Add capability modifier for capability classes

[Linyxus]

This is a proof-of-concept PR for adding a modifier for capability classes, so that we can write:

erased capability class CanIO

which will be desugared into:

erased class CanIO_$base
type CanIO = {*} CanIO_$base

This is meant to be a replacement for @capability annotation, and should fix #16725.

This PR:

• add the modifier capability
• add desugaring logic

[Linyxus]

The desugaring introduced in this PR creates problems like importing. So #16851 is probably a better solution.