Add support for generating and parsing method visibility

[AaronC81]

Is your feature request related to a problem? Please describe.
Brain Freeze currently generates RBIs for all methods, not just public ones. Fixing this would require Parlour being able to parse and consequently generate method visibility.

Describe the solution you'd like
Method RBI objects should have a visibility field, which is populated based on:

• Class context changes, like private in a class body
• Class calls with symbol arguments, like private :x, :y, :z in a class body
• Class calls with a definition as an argument, like private def ... end