Make JsAsyncGenerator fully type safe

[jedel1043]

Replaces the inner JsObject of JsAsyncGenerator with JsObject<AsyncGenerator>, which makes it fully type safe with the addition of the methods inner_next, inner_return and inner_throw.

[github-actions]

Test262 conformance changes

Test result	main count	PR count	difference
Total	52,963	52,963	0
Passed	49,935	49,935	0
Ignored	2,207	2,207	0
Failed	821	821	0
Panics	0	0	0
Conformance	94.28%	94.28%	0.00%

Tested main commit: 1f1cf0301b8a43a5893d535444279b7b11e5b3ce
Tested PR commit: 2a2c77efe96783ae3aeec6a254449ad3a6b815df
Compare commits: 1f1cf03...2a2c77e

[codecov]

Codecov Report

❌ Patch coverage is 36.53846% with 33 lines in your changes missing coverage. Please review.
✅ Project coverage is 59.01%. Comparing base (6ddc2b4) to head (2a2c77e).
⚠️ Report is 854 commits behind head on main.

Files with missing lines	Patch %	Lines
core/engine/src/builtins/async_generator/mod.rs	52.77%	17 Missing ⚠️
...ore/engine/src/object/builtins/jsasyncgenerator.rs	0.00%	16 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##             main    #5066       +/-   ##
===========================================
+ Coverage   47.24%   59.01%   +11.76%     
===========================================
  Files         476      563       +87     
  Lines       46892    62578    +15686     
===========================================
+ Hits        22154    36929    +14775     
- Misses      24738    25649      +911     

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