Fix GetSubstitution

[HalidOdat]

Fixes GetSubstitution which is used in by RegExp.prototype[@@replace] and String.prototype.replace() calls RegExp.prototype[@@replace].

Fixes the remaining ES5 tests on buil-ins/String test suite.

Because of the bug in the spec about GetSubstitution. I had to deviate from the spec, added a comment clarifying the deviation.

[github-actions]

Test262 conformance changes

Test result	main count	PR count	difference
Total	94,657	94,657	0
Passed	73,901	73,913	+12
Ignored	17,505	17,505	0
Failed	3,251	3,239	-12
Panics	0	0	0
Conformance	78.07%	78.09%	+0.01%

Fixed tests (12):

test/built-ins/String/prototype/replace/S15.5.4.11_A3_T2.js [strict mode] (previously Failed)
test/built-ins/String/prototype/replace/S15.5.4.11_A3_T2.js (previously Failed)
test/built-ins/String/prototype/replace/S15.5.4.11_A3_T1.js [strict mode] (previously Failed)
test/built-ins/String/prototype/replace/S15.5.4.11_A3_T1.js (previously Failed)
test/built-ins/String/prototype/replace/S15.5.4.11_A3_T3.js [strict mode] (previously Failed)
test/built-ins/String/prototype/replace/S15.5.4.11_A3_T3.js (previously Failed)
test/built-ins/RegExp/prototype/Symbol.replace/flags-tostring-error.js [strict mode] (previously Failed)
test/built-ins/RegExp/prototype/Symbol.replace/flags-tostring-error.js (previously Failed)
test/built-ins/RegExp/prototype/Symbol.replace/get-unicode-error.js [strict mode] (previously Failed)
test/built-ins/RegExp/prototype/Symbol.replace/get-unicode-error.js (previously Failed)
test/built-ins/RegExp/prototype/Symbol.replace/get-flags-err.js [strict mode] (previously Failed)
test/built-ins/RegExp/prototype/Symbol.replace/get-flags-err.js (previously Failed)

[codecov]

Codecov Report

Merging #2933 (682eba7) into main (d13979d) will decrease coverage by 0.03%.
The diff coverage is 86.48%.

@@            Coverage Diff             @@
##             main    #2933      +/-   ##
==========================================
- Coverage   52.09%   52.06%   -0.03%     
==========================================
  Files         435      435              
  Lines       43713    43696      -17     
==========================================
- Hits        22771    22750      -21     
- Misses      20942    20946       +4     

Impacted Files	Coverage Δ
boa_engine/src/builtins/regexp/mod.rs	63.48% <81.63%> (-0.21%)	⬇️
boa_engine/src/builtins/string/mod.rs	57.11% <96.00%> (-0.71%)	⬇️

... and 1 file with indirect coverage changes

[jedel1043]

Nice work! Really like the additional notes to clarify spec deviations :)

[nekevss]

Looks good! Seconding that the notes updates were nice.