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	52,963	52,963	0
Passed	50,735	50,737	+2
Ignored	1,426	1,426	0
Failed	802	800	-2
Panics	0	0	0
Conformance	95.79%	95.80%	+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: f5e88de558e038f0ae675a012d59917a098f44b6
Tested PR commit: 1594d05822d69d4bbe61769fe1bcab8ace1374d9
Compare commits: f5e88de...1594d05

[codecov]

Codecov Report

❌ Patch coverage is 58.00933% with 270 lines in your changes missing coverage. Please review.
✅ Project coverage is 59.59%. Comparing base (6ddc2b4) to head (1594d05).
⚠️ Report is 925 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/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 12 more

Additional details and impacted files

@@             Coverage Diff             @@
##             main    #5284       +/-   ##
===========================================
+ Coverage   47.24%   59.59%   +12.35%     
===========================================
  Files         476      589      +113     
  Lines       46892    63663    +16771     
===========================================
+ Hits        22154    37940    +15786     
- Misses      24738    25723      +985     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.