Fixes scala#22103

Subtype problems where at least one side is a type variable
representing a capture variable are canonicalized to
capturing type comparisons on the special `CapSet` for the
universe capture sets. For example, `C <: CapSet^{C^}`
becomes `CapSet^{C^} <: CapSet^{C^}`, and `A <: B` becomes
`CapSet^{A^} <: CapSet^{B^}` if both `A` and `B` are capture
variables.

Fixes scala#22103

Subtype problems where at least one side is a type variable
representing a capture variable are canonicalized to
capturing type comparisons on the special `CapSet` for the
universe capture sets. For example, `C <: CapSet^{C^}`
becomes `CapSet^{C^} <: CapSet^{C^}`, and `A <: B` becomes
`CapSet^{A^} <: CapSet^{B^}` if both `A` and `B` are capture
variables.

Fixes scala#22103

Subtype problems where at least one side is a type variable
representing a capture variable are canonicalized to
capturing type comparisons on the special `CapSet` for the
universe capture sets. For example, `C <: CapSet^{C^}`
becomes `CapSet^{C^} <: CapSet^{C^}`, and `A <: B` becomes
`CapSet^{A^} <: CapSet^{B^}` if both `A` and `B` are capture
variables.

Fixes #22103

Subtype problems where at least one side is a type variable representing
a capture variable are canonicalized to capturing type comparisons on
the special `CapSet` for the universe capture sets. For example, `C <:
CapSet^{C^}` becomes `CapSet^{C^} <: CapSet^{C^}`, and `A <: B` becomes
`CapSet^{A^} <: CapSet^{B^}` if both `A` and `B` are capture variables.

Supersedes #22183 and
#22289. This solution is overall
cleaner and does not require adding a new bit to the TypeComparer's
ApproxState.

TODOs/Issues/Questions:

- [x] Fix extension method in test
[cc-poly-varargs.scala](https://github.com/dotty-staging/dotty/blob/capture-subtyping-canon/tests/pos-custom-args/captures/cc-poly-varargs.scala).
Currently causes an infinite regress.
   - [x] Fix the aftermath
      * tests/neg-custom-args/captures/lazylists-exceptions.scala
      * tests/neg-custom-args/captures/exceptions.scala
      * tests/neg-custom-args/captures/real-try.scala
      * tests/run-custom-args/captures/colltest5
- [x] Some negative cases in test
[capture-vars-subtyping.scala](https://github.com/dotty-staging/dotty/blob/capture-subtyping-canon/tests/neg-custom-args/captures/capture-vars-subtyping.scala)
pass: `D <: E` fails, but its canonicalized form `CapSet^{D^} <:
CapSet^{E^}` now succeeds. Potential problem in the subcapturing
implementation.
- [x] ~Extend to intersection/unions `def f[C^, D^, E <: C | D, F <: C &
D](...) = ...` etc.~ Lacking good uses cases, not planned right now.
- [X] ~If we have `C^` declared in the current context, should there be
a difference between `C` vs. `C^` for subsequent mentions? We currently
do, but seems a bit too subtle for users.~ Will be addressed by a new
scheme for declaring capture variables using context bounds.