Change type of prototypes of objects to Option<JsObject>

[jedel1043]

The objective of this PR is to try to make use of the null pointer optimization of the Option type, theoretically optimizing the access of the prototype of an object.

It changes the following:

• Changes the type of prototype of objects from JsValue to JsObject
• Creates a new type alias ObjectPrototype to avoid having to read Option<Option<JsObject>> in some places. (name subject to change, maybe? I was thinking of using plain Prototype but there's already a PROTOTYPE constant, and ObjectPrototype doesn't sound as good because it could be confused with the intrinsic prototypes of constructor objects. I'd like to hear your suggestions on this)

[github-actions]

Test262 conformance changes:

Test result	main count	PR count	difference
Total	86,438	86,438	0
Passed	37,324	37,324	0
Ignored	19,022	19,022	0
Failed	30,092	30,092	0
Panics	0	0	0
Conformance	43.18%	43.18%	0.00%

[RageKnify]

It's always nice to use the type-system to ensure invariants, should result in faster and more memory efficient code.

[raskad]

Concerning the name: I think we should prefix it with Js. So JsObjectPrototype or just JsPrototype?

[raskad]

Looks good to me. Really nice idea.

[jedel1043]

Concerning the name: I think we should prefix it with Js. So JsObjectPrototype or just JsPrototype?

I like JsPrototype, it's clear and it doesn't overlap with the PROTOTYPE constant :)