+ ruby34.y: reject return in singleton class

[Earlopain]

This tracks upstream commit ruby/ruby@e642ddf

Closes #1029.

Note that in_class is now unused for 3.4 as far as I can tell. Ruby uses it in begin_definition helper but I don't think there's something equivalent with ragel here, or I just don't know about it. Kept around to keep it closer to upstream.

Tested against https://github.com/eliotsykes/real-world-rails and https://github.com/jeromedalbert/real-world-ruby-apps, got no unexpected syntax errors.

[iliabylich]

Thanks. Can't say I like that there's a version-specific shape of the context now but I don't see a better solution (branching context would most probably only make it worse), so 👍

[Earlopain]

Could try to backport this to the older parsers. But tbh that seems pretty daunting, especially when they start to deviate more and more. I think I chose a pretty easy issue to start out with otherwise, so I'm glad you didn't insist on it 😅