Provide completion for keyword arguments in method calls

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.:

```ruby
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
```