Implement window/showMessageRequest [applies PR 5492 to latest]

[smortime]

Applies @ayoub-benali's changes from https://github.com/helix-editor/helix/pull/5492 onto the latest master.

Tried and failed miserably to rebase their branch since it's 1+ year stale. Can try rebasing it again later. This unblocked me with using Metals LSP so opening this incase helps others in same situation.

All credit to Ayoub for actually implementing this and making Metals usable!

https://github.com/helix-editor/helix/issues/4699