Optimize array unshift

[yush-1018]

Add dense array fast-path for Array.prototype.unshift, same as the existing one in [shift]

Partially addresses #3407.

[github-actions]

Test262 conformance changes

Test result	main count	PR count	difference
Total	52,963	52,963	0
Passed	49,935	49,933	-2
Ignored	2,207	2,207	0
Failed	821	823	+2
Panics	0	0	0
Conformance	94.28%	94.28%	-0.00%

Broken tests (2):

test/built-ins/Array/prototype/unshift/set-length-array-length-is-non-writable.js (previously Passed)
test/built-ins/Array/prototype/unshift/set-length-array-is-frozen.js (previously Passed)

Tested main commit: 1dacd48b3356a377ba9d594d9a9ce898b519f7a6
Tested PR commit: dfc891be658ae3c5c4739c275a4d5bd07b4c08c5
Compare commits: 1dacd48...dfc891b

[codecov]

Codecov Report

❌ Patch coverage is 50.00000% with 14 lines in your changes missing coverage. Please review.
✅ Project coverage is 59.04%. Comparing base (6ddc2b4) to head (dfc891b).
⚠️ Report is 854 commits behind head on main.

Files with missing lines	Patch %	Lines
core/engine/src/builtins/array/mod.rs	50.00%	14 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##             main    #5076       +/-   ##
===========================================
+ Coverage   47.24%   59.04%   +11.80%     
===========================================
  Files         476      563       +87     
  Lines       46892    62602    +15710     
===========================================
+ Hits        22154    36964    +14810     
- Misses      24738    25638      +900     

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