Implement step 5 in RegExp constructor

[HalidOdat]

The following step was missing from our implementation, which caused some test262 test to fail.

5. Else if patternIsRegExp is true, then
    a. Let P be ? Get(pattern, "source").
    b. If flags is undefined, then
        i. Let F be ? Get(pattern, "flags").
    c. Else,
        i. Let F be flags.

[github-actions]

Test262 conformance changes

Test result	main count	PR count	difference
Total	95,574	95,574	0
Passed	75,156	75,164	+8
Ignored	19,494	19,494	0
Failed	924	916	-8
Panics	0	0	0
Conformance	78.64%	78.64%	+0.01%

Fixed tests (8):

test/built-ins/RegExp/from-regexp-like-flag-override.js [strict mode] (previously Failed)
test/built-ins/RegExp/from-regexp-like-flag-override.js (previously Failed)
test/built-ins/RegExp/from-regexp-like-get-source-err.js [strict mode] (previously Failed)
test/built-ins/RegExp/from-regexp-like-get-source-err.js (previously Failed)
test/built-ins/RegExp/from-regexp-like.js [strict mode] (previously Failed)
test/built-ins/RegExp/from-regexp-like.js (previously Failed)
test/built-ins/RegExp/from-regexp-like-get-flags-err.js [strict mode] (previously Failed)
test/built-ins/RegExp/from-regexp-like-get-flags-err.js (previously Failed)

[codecov]

Codecov Report

Patch coverage: 64.28% and project coverage change: -0.01% ⚠️

Comparison is base (25c120b) 49.80% compared to head (7884ed3) 49.80%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3305      +/-   ##
==========================================
- Coverage   49.80%   49.80%   -0.01%     
==========================================
  Files         441      441              
  Lines       43078    43083       +5     
==========================================
+ Hits        21456    21457       +1     
- Misses      21622    21626       +4     

Files Changed	Coverage Δ
boa_engine/src/builtins/regexp/mod.rs	61.91% <64.28%> (-0.53%)	⬇️

... and 1 file with indirect coverage changes

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[jedel1043]

Nice catch!