add language idris

[mtoohey31]

This pull request adds support for the Idris2 language. No grammar is included yet because I don't believe there is one (but I'm happy to be proven wrong if someone is able to find one). The only repo I could find was this one, but it's incomplete.

I have tested the language server and it works without issues.

The only thing I was unsure about is whether the filetype should be called idris or idris2, since there are breaking changes between the two languages. I lean towards idris, because:

• files of both types share the .idr extension, so we couldn't really support both even if we wanted to since there's no simple way to distinguish between the languages
• Idris 1 is not actively maintained

[archseer]

Thanks! I think that's fine, we can always rename it in the future if needed