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	53,125	53,125	0
Passed	51,072	51,074	+2
Ignored	1,482	1,482	0
Failed	571	569	-2
Panics	0	0	0
Conformance	96.14%	96.14%	+0.00%

Fixed tests (2):

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

Tested main commit: 8a1e8fe07f626f7a067afc2c9885d5d87de4bb5d
Tested PR commit: c9edeabcac795ff66bb4af55df813bce8b61d603
Compare commits: 8a1e8fe...c9edeab

[codecov]

Codecov Report

❌ Patch coverage is 57.85381% with 271 lines in your changes missing coverage. Please review.
✅ Project coverage is 62.51%. Comparing base (6ddc2b4) to head (c9edeab).
⚠️ Report is 985 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	65.51%	50 Missing ⚠️
core/engine/src/builtins/proxy/mod.rs	11.42%	31 Missing ⚠️
core/engine/src/object/property_map.rs	54.41%	31 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 11 more

Additional details and impacted files

@@             Coverage Diff             @@
##             main    #5284       +/-   ##
===========================================
+ Coverage   47.24%   62.51%   +15.27%     
===========================================
  Files         476      535       +59     
  Lines       46892    59272    +12380     
===========================================
+ Hits        22154    37055    +14901     
+ Misses      24738    22217     -2521     

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