Add instructions for helix editor

[sebffischer]

I think it is currently impossible, but will soon be possible to use air with the helix editor.
The problem currently is that the content of the file is passed to the formatter via stdin, but air requires the filenames (see the issue in helix here: helix-editor/helix#3596).

However, there is an open Pull Request in helix that gives access to the file name to the formatter, so once this is merged I think air should be usable with helix and I could add instructions.

On a side-note: are there some "good first issue" issues that can be tackled here? Would love to contribute.

[DavisVaughan]

The Air command line tool offers a full language server, which Helix seems to support. We do plan to add stdin support to the air format command, but you'll probably have better luck with air language-server set up in the same way as the ruff guide
https://docs.astral.sh/ruff/editors/setup/#helix

(I haven't tried this though)

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are there some "good first issue" issues

Nothing quite yet, still working through some larger features that might result in major infrastructure rewrites, particularly in the lsp server. But we expect that to stabilize in a month or so and then we would love to have more contributors!