Ignore capture sets in certain cases of subtyping

[bracevac]

Fixes #22103

If C is a capture-set variable, then a comparison C <: CapSet^{C^} would previously fail in the TypeComparer due to a level mismatch resulting in the test CapSet^ <: CapSet.

For such cases, we now ignore the LHS capture annotation. This is controlled by a new bit in the ApproxState of the TypeComparer.

[noti0na1]

The extension methods above can be tested as well. I remember they have similar issues.

[noti0na1]

We should have standalone file to test the subtyping:

def test[C^] = 
  val a: C = ???
  val b: CapSet^{C^} = a
  val c: C = b
  val d: CapSet^{C^, ...} = a
  ...

We also want to test nested types are still working:

def test[C^, D^] = 
  A[C] <:< A[CapSet^{C^}] // ok
  A[C] <:< A[CapSet^{D^}] // error
  A[C]^{D^} <:< A[CapSet^{D^}]^{D^} // error