Support extension methods on erased types

[nicolasstucki]

Compiler version

3.0.0-RC1

Minimized example

//  import language.experimental.erasedTerms
import scala.compiletime.erasedValue
type UnivEq[A]
object UnivEq:
  erased def force[A]: UnivEq[A] = erasedValue
  extension [A](erased proof: UnivEq[A])
    inline def univEq(a: A, b: A): Boolean =
      a == b

With -Yerased-terms or the commented line (after #11695) is merged.

Output

-- [E040] Syntax Error: Foo.scala:6:16 -----------------------------------------
6 |  extension [A](erased proof: UnivEq[A])
  |                ^^^^^^
  |                an identifier expected, but 'erased' found

Expectation

It should compile