Support endless method definitions

[eregon]

Extracted from #2733

The relevant changes in CRuby are:

• https://bugs.ruby-lang.org/issues/16746
• https://bugs.ruby-lang.org/issues/17398

and the associated revisions are:

• https://bugs.ruby-lang.org/projects/ruby-master/repository/git/revisions/e8f53692ca7d65c92bde6d5bc6a1aea492915d9f/diff
• https://bugs.ruby-lang.org/projects/ruby-master/repository/git/revisions/31794d2e733e081e4e221f27aff6380393981681

There seems to be no changes in the lexer, good.

This seems used frequently in applications, so we should try to support it soon.

We should also check if JRuby master supports them, and if so we can copy the relevant parts of https://github.com/jruby/jruby/blob/master/core/src/main/java/org/jruby/parser/RubyParser.y then.

We will wait for Prism/YARP adoption to fix this :#3117

[eregon]

Will be done for 24.0.