Generically handle all method visibility modifiers

[reese]

Resolves #363

Previously we explicitly listed out all "visibility modifiers" like private, private_class_method, etc. to avoid adding extra newlines to them when rendering "method modifiers" like sigs. However, there will probably always be some we could be missing this way (right now, this includes things like module_function and ruby2_keywords), so this PR changes it such that we generically this for anything of the shape

end
<whatever> def func

which seems more reasonable, especially if a user defined their own special method (like Sorbet's custom package_private).