Support "Endless method definition" syntax for Ruby 2.8 (3.0)

[koic]

This PR supports "Endless method definition" syntax for Ruby 2.8 (3.0).
Parser gem supports this syntax by whitequark/parser#676.

Ref: https://bugs.ruby-lang.org/issues/16746

[bbatsov]

I think we should also add an endless? predicate to the node extension.

[marcandre]

I commented in whitequark/parser#676 that I hope that we can get a simple def (and also in https://bugs.ruby-lang.org/issues/16746 that I hope this is reverted...)
I think we should wait for this feature to be confirmed, or at at least for parser's response?

[koic]

Thanks for the discussion at Parser. I thought that it could be introduced to RubyCop AST experimentally, referring to the fact that Ruby Next supports it.
https://github.com/ruby-next/ruby-next

I think we should wait for this feature to be confirmed, or at at least for parser's response?

At least I think we can wait for Parser's response.

https://bugs.ruby-lang.org/issues/16746 that I hope this is reverted...

To be honest, I'm also wondering about adding this syntax to Ruby 😅

Apart from that I added endless? predicate. Thank you.

[marcandre]

So parser will output def after all. We could add an endless? predicate that would check the presence of loc.end

[koic]

Yeah, I will update this PR when Parser 2.7.1.5 (or Parser 2.7.2.0) is released. Thank you for your kindness advice!

[koic]

This PR will be closed due to prefer #129. Thank you @marcandre!

[marcandre]

This PR will be closed due to prefer #129. Thank you @marcandre!

My pleasure 😄