[Merged by Bors] - Fix String.prototype.substr()

[HalidOdat]

Fixes the remaining failing tests on String.prototype.substr()

It changes the following:

• Fixes the remaining failing tests on `String.prototype.substr()
• Move substr under the "annex-b" feature flag.

[github-actions]

Test262 conformance changes

Test result	main count	PR count	difference
Total	94,781	94,781	0
Passed	71,383	71,385	+2
Ignored	17,824	17,824	0
Failed	5,574	5,572	-2
Panics	0	0	0
Conformance	75.31%	75.32%	+0.00%

Fixed tests (2):

test/annexB/built-ins/String/prototype/substr/start-and-length-as-numbers.js [strict mode] (previously Failed)
test/annexB/built-ins/String/prototype/substr/start-and-length-as-numbers.js (previously Failed)

[codecov]

Codecov Report

Merging #2805 (f09c45d) into main (24a4770) will decrease coverage by 0.01%.
The diff coverage is 4.00%.

@@            Coverage Diff             @@
##             main    #2805      +/-   ##
==========================================
- Coverage   51.30%   51.29%   -0.01%     
==========================================
  Files         416      416              
  Lines       41193    41197       +4     
==========================================
  Hits        21133    21133              
- Misses      20060    20064       +4     

Impacted Files	Coverage Δ
boa_engine/src/builtins/string/mod.rs	57.06% <4.00%> (-0.24%)	⬇️

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[jedel1043]

Good catch!

[Razican]

bors r+

[bors]

Pull request successfully merged into main.

Build succeeded:

• Build and Test (macos-latest)
• Build and Test (windows-latest)
• Coverage
• Misc