feat(gc): make WeakMap::get return a reference instead of cloning

[shruti2522]

This Pull Request fixes/closes #{issue_num}.

It changes the following:

WeakMap::get now returns an Option<GcRef<'_, V>> instead of Option<V>

[github-actions]

Test262 conformance changes

Test result	main count	PR count	difference
Total	52,963	52,963	0
Passed	49,687	49,687	0
Ignored	2,262	2,262	0
Failed	1,014	1,014	0
Panics	0	0	0
Conformance	93.81%	93.81%	0.00%

Tested main commit: b3d820369072123ded8defc927405b99b9012648
Tested PR commit: f69557f44c8ca23d8154f35edf68bf27d397131e
Compare commits: b3d8203...f69557f

[jedel1043]

This is unsound. If you get a reference to the inner value of the ephemeron and a garbage collection occurs, you could be left with a reference to deallocated data.