Surface method signatures to the Ruby API

[vinistock]

We need to be able to read method signatures to provide signature help in the Ruby LSP.

There's one interesting caveat to investigate. We don't eagerly resolve method aliases and their signatures are attached to the method they target. For example:

class Parent
  def foo(a, b); end
end

class Child < Parent
  alias_method :new_foo, :foo
end

The signature for new_foo should be [PositionalRequired(a), PositionalRequired(b)] coming from the original foo.

Investigate if we can resolve method aliases lazily to return signatures (and consider splitting the work into separate PRs if necessary).

[vinistock]

Another thing: don't forget about method overrides:

class Foo
  def bar(a); end
  def bar(a, b); end
end

Signatures should be exposed from definitions (i.e.: MethodDefinition#signatures).

[soutaro]

• Want to do something for RBS untyped method (?) -> T (but how?)
• Method aliases to inherited methods should work -- should return signatures from the inherited methods.