Nowarn receiver of extension taking params

[som-snytt]

Fixes #23349

Extensions are regular methods, but the "receiver" parameter is exempt from the unused check. By error, previously x.m was exempt but not x.f(y).

(The reason for the exemption may be that the method expressed as a member of the receiver type may make no reference to this, without warning.)

Check for parameters used only in default arg expressions. A parameter may be aliased in a default arg getter method, so a usage of that getter param counts as a usage of the method param (or class param). Defaults of class params are found in the class companion.

[som-snytt]

There was a sad regression test that is nice to fix at https://github.com/scala/scala3/blob/3.7.1/tests/warn/i15503e.scala#L95

  def f(i: Int)(j: Int = i * 2) = j // warn I guess

Maybe that is called a "concession test".