Decorators normative updates

[rbuckton]

This changes our ES decorators emit to match with two normative updates to the Decorators proposal:

• The this receiver should be preserved when a decorator is called, i.e., when the decorator @foo.bar is invoked, foo should be the this of the bar method.
• Static accessor fields should use the class reference rather than this when accessing the backing field.

See tc39/proposal-decorators#499 for more information.

Fixes #53752