Provide completion for keyword arguments in method calls

[vinistock]

If we know exactly which method the user is currently typing, we can provide completion for the keyword arguments required to invoke that method.

E.g.:

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

  def baz
    baz()
      #^^ when the cursor is here we can show `a` and `b` as possible symbol completions
  end
end