Return optional value in to_json functions (fixes #1064)

[fermian]

This Pull Request fixes/closes #1064.

It changes the following:

• Makes Value::to_json, GcObject::to_json return an optional JSONValue. If the value cannot be converted into a JSON value, the functions return None.

[Razican]

Test262 conformance changes:

Test result	master count	PR count	difference
Total	78,497	78,497	0
Passed	25,499	25,501	+2
Ignored	15,587	15,587	0
Failed	37,411	37,409	-2
Panics	18	14	-4
Conformance	32.48	32.49	+0.00%

Benchmarks

┌─────────┬──────────────────────────────────────────────┬─────────────────────┬──────────────────────┬────────────┐
│ (index) │                     name                     │   changesDuration   │    masterDuration    │ difference │
├─────────┼──────────────────────────────────────────────┼─────────────────────┼──────────────────────┼────────────┤
│    0    │     'Arithmetic operations (Execution)'      │ '**312.1±5.97ns**'  │    '314.7±5.83ns'    │  '-0.99'   │
│    1    │        'Arithmetic operations (Full)'        │   '244.1±5.81µs'    │  '**241.7±3.30µs**'  │   '+1.0'   │
│    2    │          'Array access (Execution)'          │  '**5.9±0.30µs**'   │     '5.9±0.19µs'     │  '-0.99'   │
│    3    │            'Array access (Full)'             │   '263.1±9.89µs'    │  '**257.5±5.38µs**'  │   '+2.0'   │
│    4    │         'Array creation (Execution)'         │    '2.6±0.10ms'     │     '2.6±0.09ms'     │   '0.0'    │
│    5    │           'Array creation (Full)'            │    '2.6±0.07ms'     │     '2.6±0.04ms'     │   '0.0'    │
│    6    │           'Array pop (Execution)'            │ '**823.3±13.16µs**' │   '842.9±45.96µs'    │   '-2.0'   │
│    7    │              'Array pop (Full)'              │  '1220.5±43.30µs'   │ '**1207.2±32.38µs**' │   '+1.0'   │
│    8    │     'Boolean Object Access (Execution)'      │    '5.2±0.43µs'     │   '**5.0±0.15µs**'   │   '+5.0'   │
│    9    │        'Boolean Object Access (Full)'        │   '256.2±8.04µs'    │    '256.0±4.92µs'    │   '0.0'    │
│   10    │            'Clean js (Execution)'            │ '**607.4±19.37µs**' │   '618.1±15.52µs'    │   '-2.0'   │
│   11    │              'Clean js (Full)'               │   '886.9±30.40µs'   │ '**870.3±15.63µs**'  │   '+2.0'   │
│   12    │             'Clean js (Parser)'              │    '40.6±1.06µs'    │  '**40.1±0.70µs**'   │   '+1.0'   │
│   13    │                'Create Realm'                │ '**404.6±15.41ns**' │    '419.7±8.71ns'    │   '-3.8'   │
│   14    │ 'Dynamic Object Property Access (Execution)' │    '4.8±0.13µs'     │     '4.8±0.14µs'     │   '0.0'    │
│   15    │   'Dynamic Object Property Access (Full)'    │   '263.0±31.09µs'   │  '**256.7±4.81µs**'  │   '+2.0'   │
│   16    │            'Expression (Parser)'             │  '**6.1±0.09µs**'   │     '6.5±0.18µs'     │   '-4.8'   │
│   17    │           'Fibonacci (Execution)'            │ '**774.4±14.13µs**' │   '780.7±16.34µs'    │  '-0.99'   │
│   18    │              'Fibonacci (Full)'              │  '1008.5±34.38µs'   │   '1012.3±43.14µs'   │   '0.0'    │
│   19    │            'For loop (Execution)'            │    '20.3±0.68µs'    │    '20.2±0.38µs'     │   '0.0'    │
│   20    │              'For loop (Full)'               │   '275.8±14.36µs'   │  '**270.0±9.04µs**'  │   '+2.0'   │
│   21    │             'For loop (Parser)'              │    '19.0±0.62µs'    │    '18.9±0.51µs'     │   '0.0'    │
│   22    │           'Goal Symbols (Parser)'            │    '13.2±0.28µs'    │    '13.2±0.40µs'     │   '0.0'    │
│   23    │            'Hello World (Parser)'            │    '3.5±0.13µs'     │     '3.6±0.06µs'     │   '0.0'    │
│   24    │             'Long file (Parser)'             │ '**692.0±22.38ns**' │   '699.3±16.00ns'    │  '-0.99'   │
│   25    │            'Mini js (Execution)'             │   '553.5±33.67µs'   │ '**548.4±12.50µs**'  │   '+1.0'   │
│   26    │               'Mini js (Full)'               │   '807.0±18.57µs'   │   '803.8±15.03µs'    │   '0.0'    │
│   27    │              'Mini js (Parser)'              │    '35.4±0.93µs'    │  '**35.2±0.70µs**'   │   '+1.0'   │
│   28    │      'Number Object Access (Execution)'      │    '4.0±0.12µs'     │   '**3.8±0.12µs**'   │   '+6.0'   │
│   29    │        'Number Object Access (Full)'         │   '257.2±9.82µs'    │ '**253.7±13.61µs**'  │   '+1.0'   │
│   30    │        'Object Creation (Execution)'         │  '**4.1±0.15µs**'   │     '4.1±0.17µs'     │  '-0.99'   │
│   31    │           'Object Creation (Full)'           │   '255.4±10.68µs'   │    '255.2±8.40µs'    │   '0.0'    │
│   32    │             'RegExp (Execution)'             │  '**9.1±0.51µs**'   │     '9.3±0.33µs'     │   '-2.9'   │
│   33    │               'RegExp (Full)'                │   '262.1±5.45µs'    │  '**258.6±7.71µs**'  │   '+1.0'   │
│   34    │         'RegExp Literal (Execution)'         │  '**10.2±0.18µs**'  │    '10.5±0.33µs'     │   '-2.9'   │
│   35    │           'RegExp Literal (Full)'            │   '267.2±15.32µs'   │    '266.4±7.80µs'    │   '0.0'    │
│   36    │    'RegExp Literal Creation (Execution)'     │    '8.9±0.32µs'     │     '9.0±0.18µs'     │   '0.0'    │
│   37    │       'RegExp Literal Creation (Full)'       │   '263.2±14.31µs'   │  '**259.3±7.92µs**'  │   '+2.0'   │
│   38    │ 'Static Object Property Access (Execution)'  │    '4.3±0.13µs'     │     '4.3±0.12µs'     │   '0.0'    │
│   39    │    'Static Object Property Access (Full)'    │   '260.6±8.57µs'    │  '**253.3±5.20µs**'  │   '+3.0'   │
│   40    │      'String Object Access (Execution)'      │    '7.0±0.18µs'     │   '**6.8±0.18µs**'   │   '+3.0'   │
│   41    │        'String Object Access (Full)'         │   '261.1±10.29µs'   │  '**259.3±7.44µs**'  │   '+1.0'   │
│   42    │       'String comparison (Execution)'        │  '**6.0±0.16µs**'   │     '6.1±0.15µs'     │   '-2.0'   │
│   43    │          'String comparison (Full)'          │ '**259.1±6.43µs**'  │   '260.6±11.41µs'    │  '-0.99'   │
│   44    │      'String concatenation (Execution)'      │  '**4.9±0.19µs**'   │     '5.0±0.25µs'     │  '-0.99'   │
│   45    │        'String concatenation (Full)'         │   '253.5±9.56µs'    │  '**250.5±8.28µs**'  │   '+1.0'   │
│   46    │          'String copy (Execution)'           │  '**3.7±0.13µs**'   │     '3.7±0.09µs'     │  '-0.99'   │
│   47    │             'String copy (Full)'             │   '245.3±5.94µs'    │    '245.3±5.54µs'    │   '0.0'    │
│   48    │            'Symbols (Execution)'             │    '3.3±0.16µs'     │   '**3.2±0.10µs**'   │   '+2.0'   │
│   49    │               'Symbols (Full)'               │   '235.9±7.46µs'    │  '**232.2±5.05µs**'  │   '+2.0'   │
│   50    │                      ''                      │      undefined      │      undefined       │   '+NaN'   │
└─────────┴──────────────────────────────────────────────┴─────────────────────┴──────────────────────┴────────────┘

Benchmarks and test results look good.