Add a specific error message for local final defs

[mbovel]

Fixes #17579.

[odersky]

The main thing is to remove or motivate the restriction why GIVEN is excluded. Otherwise LGTM.

[odersky]

Why a different error message here?

[odersky]

I see why: since private is also not a local modifier token.

[odersky]

This means we could generalize the solution. We could always parse all tokens and then flag those that are not legal at this point. But this could be another PR.

[mbovel]

The main thing is to remove or motivate the restriction why GIVEN is excluded.

We thought we should not emit an error in this case, as final has a different meaning for classes, which given can be translated to. It makes sense to define a final class Foo in a local scope for example.

And there is an existing warning for final given:

trait Foo:
  def foo: Int

@main def f() =
  final given Foo with
    def foo = 42

➜  ~/scala-snippets-5 scala-cli -Xprint:typer final_given.scala
Compiling project (Scala 3.4.1, JVM (17))
[warn] ./final_given.scala:7:3
[warn] Modifier final is redundant for this definition
[warn]   final given Foo with
[warn]   ^^^^^
[[syntax trees at end of                     typer]] // /Users/mbovel/scala-snippets-5/final_given.scala
package final_given {
  trait Foo() extends Object {
    def foo: Int
  }
  final lazy module val final_given$package: final_given.final_given$package =
    new final_given.final_given$package()
  final module class final_given$package() extends Object() {
    this: final_given.final_given$package.type =>
    @main def f(): Unit =
      {
        final lazy module given val given_Foo: given_Foo = new given_Foo()
        final module class given_Foo() extends Object(), final_given.Foo {
          this: given_Foo.type =>
          def foo: Int = 42
        }
...

Does that make sense? Should I had a comment to explain this where we through the error?

[mbovel]

I rebased on main, and added comments in the parser and in the test case.