Add formatter elm-format for Elm

[heyakyra]

No description provided.

[the-mikedavis]

I'm not sure we should add an external formatter for Elm: elm-format would be used instead of the LSP auto-formatting (which also uses elm-format but computes the diffs) when installed. There isn't currently a way to prefer the LSP formatting but fall back to the external formatter.

[kirawi]

Helix will generate a diff when using an external formatter, but it may not be as efficient on larger files because it will fallback to a less efficient fast path after a 1 second timeout.

[heyakyra]

Ah, thank you. I must be hitting this elm-tooling/elm-language-server#692

[michaeljones]

@heyakyra - is it working smoothly for you at the moment? I seem to be in a state where the language server works for a few minutes and then just times out on every request. I can't explain it. Probably doing some silly. I'd be interested to hear confirmation that it is working for someone else though.