Warn if a type parameter shadows a non-parameter

[b-studios]

This tries to fix #368 by emitting a warning in case a type parameter shadows an outer type, which is not a type parameter (since this sometimes is desired and could lead to too many false negatives).

An example can be seen here:

effekt/examples/neg/namer/shadowing-types.effekt

Lines 1 to 17 in 2c43c68

	effect Foo[A](x: A): A
	interface Bar {
	def op[B](b: B): Unit
	}
	def bar = new Bar {
	def op[Int](n) = () // WARN shadows
	}
	def main(): Int =
	try {
	//...
	0
	} with Foo[Int] { // WARN shadows
	(x: Int) => 12
	}

[b-studios]

TBH while going through this code I was tempted to refactor Scope to be an enum. This code is older than Scala 3 and writing it in a functional style might be clearer.