Refactor Class API

[jedel1043]

Right now our Class and ClassBuilder API is a lot stricter than its equivalent ConstructorBuilder, which makes ClassBuilder a bit clunkier to use. Ideally we should integrate all ConstructorBuilder's APIs with ClassBuilder, and also add some more features to make ClassBuilder a bit nicer to use. Specifically:

• Allow using any PropertyKey as the name of a method. (Introduce a Class map #3315)
• Allow changing the base object of the generated constructor object.
• Allow deriving from other classes.
• Integrate a "class map" of sorts to easily access the registered classes (Possibly using Implement [[HostDefined]] field on Realms #2952) (Introduce a Class map #3315).
• Allow modifying the JsObject wrapping the NativeObject constructed.
• Change the argument name this to new_target and document that this is not the constructed object, but the parent constructor.
• Allow creating an object from the raw Class data (Class::from_data maybe?)