Extend Signatures to have multiple type parameter clauses

[tanishiking]

In Scala3, there could be a MethodSignature that has multiple type parameter clauses.
For example, in the following code snippet, the extension method map has two type parameter clauses [T] and [U].

extension [T](t: F[T]) def map[U](f: T => U): F[U]

However, the current SemanticDB schema is incapable of expressing multiple type parameter clauses in Signatures https://scalameta.org/docs/semanticdb/specification.html#signature

To work around this problem, in the current implementation, we express such type param clauses by merging into one type param clause:
map[T, U] instead of map[T][U] scala/scala3#12885

---

This workaround could be a problem when U is a dependent type based on T, and this signature is inaccurate.

We can't pretend that F[T][U] is F[T, U] as that breaks the arity of F, this will need a new semanticdb message to support, dotty captures this in TypeRef which just has the prefix and symbol fields, and then AppliedType has a tycon field, the type being applied and could be a nested AppliedType, and the args field
scala/scala3#12885 (comment)

We should extend the signature so they can have multiple type parameter clauses (however this requires a breaking change in SemanticDB schema), or maybe we should change the representation of extension methods in Scala3 so we can express accurate signature in SemanticDB.