Add completed PropertyDescriptor type

[HalidOdat]

Refactors property descriptor handling by replacing the builder-style PropertyDescriptor with a more explicit CompletePropertyDescriptor enum. This makes things clearer and more type-safe. The spec always stores complete property descriptors in objects.

Since descriptors are always complete, this allows us to skip some checks and may allow the compiler to better optimize the code. Additionally this also reduces the memory usage for sparse generic arrays, CompletePropertyDescriptor is 24 bytes vs. PropertyDescriptor 40 bytes (64bit arch).

[github-actions]

Test262 conformance changes

Test result	main count	PR count	difference
Total	53,578	53,578	0
Passed	51,426	51,428	+2
Ignored	1,648	1,648	0
Failed	504	502	-2
Panics	0	0	0
Conformance	95.98%	95.99%	+0.00%

Fixed tests (2):

test/staging/sm/Promise/promise-species.js (previously Failed)
test/staging/sm/Symbol/species.js (previously Failed)

Tested main commit: 665f03924a54e5162be227e7e909612e36f6e35a
Tested PR commit: 47d60cba56a5b513a72c368fa676c700db0daa32
Compare commits: 665f039...47d60cb

[codecov]

Codecov Report

❌ Patch coverage is 58.69565% with 266 lines in your changes missing coverage. Please review.
✅ Project coverage is 62.77%. Comparing base (6ddc2b4) to head (47d60cb).
⚠️ Report is 1050 commits behind head on main.

Files with missing lines	Patch %	Lines
core/engine/src/object/internal_methods/mod.rs	66.46%	56 Missing ⚠️
core/engine/src/property/mod.rs	67.58%	47 Missing ⚠️
core/engine/src/object/property_map.rs	54.41%	31 Missing ⚠️
core/engine/src/builtins/proxy/mod.rs	20.00%	28 Missing ⚠️
core/engine/src/object/mod.rs	64.06%	23 Missing ⚠️
core/engine/src/value/display/object.rs	31.81%	15 Missing ⚠️
core/engine/src/builtins/builder.rs	74.35%	10 Missing ⚠️
core/engine/src/module/namespace.rs	0.00%	9 Missing ⚠️
core/engine/src/value/display/array.rs	25.00%	9 Missing ⚠️
core/engine/src/object/internal_methods/string.rs	14.28%	6 Missing ⚠️
... and 12 more

Additional details and impacted files

@@             Coverage Diff             @@
##             main    #5284       +/-   ##
===========================================
+ Coverage   47.24%   62.77%   +15.52%     
===========================================
  Files         476      536       +60     
  Lines       46892    60498    +13606     
===========================================
+ Hits        22154    37975    +15821     
+ Misses      24738    22523     -2215     

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[jedel1043]

Mostly looks great! I just have a suggestion to avoid introducing more panics in the engine.

[jedel1043]

You might need to bump to v0.23 since removing fill_with is technically a breaking change, but it might just be possible to preserve the original public API by restricting the usage of fill_with to only incomplete descriptors, and only allow PropertyDescriptor::fill_with