Add SparseElement variant for simple sparse array storage

[HalidOdat]

Introduce a new SparseElement variant to IndexedProperties that stores simple values (data descriptors with all flags true) in a hash map without full property descriptors. This optimizes memory usage for sparse arrays.

[github-actions]

Test262 conformance changes

Test result	main count	PR count	difference
Total	50,747	50,747	0
Passed	47,877	47,877	0
Ignored	2,060	2,060	0
Failed	810	810	0
Panics	0	0	0
Conformance	94.34%	94.34%	0.00%

[codecov]

Codecov Report

❌ Patch coverage is 47.20000% with 66 lines in your changes missing coverage. Please review.
✅ Project coverage is 57.13%. Comparing base (6ddc2b4) to head (137e840).
⚠️ Report is 605 commits behind head on main.

Files with missing lines	Patch %	Lines
core/engine/src/object/property_map.rs	48.27%	60 Missing ⚠️
cli/src/debug/object.rs	0.00%	5 Missing ⚠️
core/engine/src/value/mod.rs	75.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4543      +/-   ##
==========================================
+ Coverage   47.24%   57.13%   +9.88%     
==========================================
  Files         476      504      +28     
  Lines       46892    57599   +10707     
==========================================
+ Hits        22154    32908   +10754     
+ Misses      24738    24691      -47     

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