feat(engine): add JsArrayBuffer::to_vec for copying bytes

[alienx5499]

This Pull Request fixes/closes #4974.

It changes the following:

• Add JsArrayBuffer::to_vec(&self) -> Option<Vec<u8>> helper to copy buffer contents into a fresh Vec<u8> while returning None when the buffer has been detached
• Add a doc comment and example for JsArrayBuffer::to_vec, including both normal and detached cases
• Add small unit tests (array_buffer_to_vec_roundtrip_and_detach and array_buffer_to_vec_empty) to cover round-trip, detach, and empty-buffer behavior

Testing:

• cargo test -p boa_engine (including the new unit tests and doctest)

[github-actions]

Test262 conformance changes

Test result	main count	PR count	difference
Total	52,963	52,963	0
Passed	49,713	49,713	0
Ignored	2,262	2,262	0
Failed	988	988	0
Panics	0	0	0
Conformance	93.86%	93.86%	0.00%

Tested main commit: 683d483bd03eba9f644a71d023704e35be32974f
Tested PR commit: ee8be0b422146623b8dbf3d0cff7f74e82d2d722
Compare commits: 683d483...ee8be0b

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 58.35%. Comparing base (6ddc2b4) to head (ee8be0b).
⚠️ Report is 796 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff             @@
##             main    #4976       +/-   ##
===========================================
+ Coverage   47.24%   58.35%   +11.11%     
===========================================
  Files         476      556       +80     
  Lines       46892    61153    +14261     
===========================================
+ Hits        22154    35688    +13534     
- Misses      24738    25465      +727     

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[alienx5499]

Hi @jedel1043
This PR is ready for review. Would appreciate your feedback when you have time.
Thanks!

[jedel1043]

LGTM!