Add support for __END__ in RBS files

[sampersand]

One thing I really like about Ruby is the fact that you can type \n__END__\n and it'll comment out the rest of the file. I use it very extensively, from testing quick snippets, to commenting out code i'm replacing, to finding where the missing end originated from.

I'd really like if we could support \n__END__\n in RBS. It'd act just like Ruby's (except for no DATA) and would "comment out" the rest of the file.

[sampersand]

Moved ot an issue: #1646