Implement non-erased JsObjects

[jedel1043]

This PR implements non-erased JsObjects aka JsObject<T>. This allows accessing the inner data of an object without having to call downcast_ref/mut every time, which is a big ergonomics win that should avoid calling expect a lot of times.

This is an MVP that doesn't support calling internal methods using a JsObject<T>; adding support for that would require a slight redesign on the API. However, the current MVP is enough to support most of the use cases of the API in our builtin wrappers.

This also uses the new API to improve the JsArrayBuffer wrapper on some parts, but it still needs some more work that can be done on subsequent PRs.

Thanks to @HalidOdat for nerd-sniping me into implementing this 🤣

[github-actions]

Test262 conformance changes

Test result	main count	PR count	difference
Total	95,960	95,960	0
Passed	76,594	76,594	0
Ignored	18,477	18,477	0
Failed	889	889	0
Panics	0	0	0
Conformance	79.82%	79.82%	0.00%

[codecov]

Codecov Report

Attention: 72 lines in your changes are missing coverage. Please review.

Comparison is base (6be4296) 47.46% compared to head (7c1fa18) 47.43%.

Files	Patch %	Lines
core/engine/src/object/jsobject.rs	54.28%	32 Missing ⚠️
core/engine/src/builtins/typed_array/builtin.rs	0.00%	16 Missing ⚠️
core/engine/src/object/builtins/jsarraybuffer.rs	0.00%	15 Missing ⚠️
core/engine/src/object/builtins/jsdataview.rs	0.00%	4 Missing ⚠️
core/engine/src/builtins/array_buffer/mod.rs	0.00%	3 Missing ⚠️
core/engine/src/builtins/array_buffer/utils.rs	0.00%	1 Missing ⚠️
core/gc/src/pointers/ephemeron.rs	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3618      +/-   ##
==========================================
- Coverage   47.46%   47.43%   -0.04%     
==========================================
  Files         475      476       +1     
  Lines       46044    46082      +38     
==========================================
+ Hits        21855    21857       +2     
- Misses      24189    24225      +36     

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[HalidOdat]

Great work @jedel1043! :)

[nekevss]

Looks good to me 😄