Fix regexp toString method

[raskad]

This Pull Request changes the following:

• Fix regexp toString method to honor the spec.

[github-actions]

Test262 conformance changes

Test result	main count	PR count	difference
Total	95,960	95,960	0
Passed	76,588	76,594	+6
Ignored	18,477	18,477	0
Failed	895	889	-6
Panics	0	0	0
Conformance	79.81%	79.82%	+0.01%

Fixed tests (6):

test/annexB/built-ins/RegExp/prototype/compile/pattern-string-invalid-u.js [strict mode] (previously Failed)
test/annexB/built-ins/RegExp/prototype/compile/pattern-string-invalid-u.js (previously Failed)
test/annexB/built-ins/RegExp/prototype/compile/flags-string-invalid.js [strict mode] (previously Failed)
test/annexB/built-ins/RegExp/prototype/compile/flags-string-invalid.js (previously Failed)
test/annexB/built-ins/RegExp/prototype/compile/pattern-string-invalid.js [strict mode] (previously Failed)
test/annexB/built-ins/RegExp/prototype/compile/pattern-string-invalid.js (previously Failed)

[codecov]

Codecov Report

Attention: 2 lines in your changes are missing coverage. Please review.

Comparison is base (0ed7da5) 47.33% compared to head (a1a53aa) 47.36%.
Report is 2 commits behind head on main.

Files	Patch %	Lines
core/engine/src/builtins/regexp/mod.rs	71.42%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3608      +/-   ##
==========================================
+ Coverage   47.33%   47.36%   +0.03%     
==========================================
  Files         475      475              
  Lines       46138    46137       -1     
==========================================
+ Hits        21839    21853      +14     
+ Misses      24299    24284      -15     

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[HalidOdat]

Nice catch!