Semi-Fix scaladoc of extensions methods

[Sporarum]

This changes the heuristic used by scaladoc to one I was not able to fault (smarter people might)
(à propos, the main idea behind the heuristic was @smarter's)

Note that it is not enough for tests to pass, as unwrapMemberInfo is also incorrect, but I do not have the time to fix it

The commits are neatly split and annotated, so it should be relatively easy to understand what I did

I left some commented printlns, they highlight that the new heuristic works correctly, and how unwrapMemberInfo is incorrect, for example with f1 from the rehabilitated test extensionParams:
file:

extension [A](a: A)(using Int)
  def f1[B](b: B): (A, B) 

cleaned up output:

f1
ExtensionPart:
List(type A)
List(val a: A)
List(val x$2: scala.Int)
NonExtensionPart:
List(type B)
List(val b: B)

f1
MemberInfo:
List(Map(b -> TypeParamRef(B)))
extended...:
List(List(ValDef(a,Ident(A),EmptyTree)), List(ValDef(x$2,Ident(Int),EmptyTree)))

[Sporarum]

This assert seems to be causing some of the tests to fail, but I don't understand why

[Sporarum]

extendedSymbol should probably also be updated, as it implicitly uses the old heuristic

[Sporarum]

(actually no, it doesn't ...)
I noticed @rochala's work only after sadly, I'll leave those commits here for now, as those changes might still be useful to scaladoc methods with multiple type param clauses (see: #14019)