Handle RBS files

[vinistock]

If we're going to allow for typechecking addons, such as a Steep addon, we need to be able to handle RBS files. Many of Steep's features happen inside RBS files, so it's a requirement for an addon to fully work.

[github-actions]

This issue is being marked as stale because there was no activity in the last 2 months

[ybiquitous]

Hi @vinistock, I guess supporting RBS syntax highlighting is a great first step here, but what do you think?

At this point, @soutaro's (RBS author) VSCode RBS Syntax extension is probably the best choice. This extension's syntax definition is used for GitHub (see github-linguist/linguist#6369).

[vinistock]

Hey, @ybiquitous! Soutaro and I actually discussed this during RubyKaigi. I agree we can definitely start with grammar support.

And then there are other simple features which should be straight forward to implement in the server, like folding ranges and document symbol.

[ybiquitous]

Oh, this year's RubyKaigi was a great discussion forum to improve Ruby DX! 😁
I'm so happy to hear about your positive response. I am looking forward to the RBS support!

[andyw8]

Starting point: #2775