8767: implement range formatting r=matklad a=euclio
Fixes#7580.
This PR implements the `textDocument/rangeFormatting` request using `rustfmt`'s `--file-lines` option.
Still needs some tests. What I want to know is how I should handle the instability of the `--file-lines` option. It's still unstable in rustfmt, so it's only available on nightly, and needs a special flag to enable. Is there a way for `rust-analyzer` to detect if it's using nightly rustfmt, or for users to opt-in?
Co-authored-by: Andy Russell <arussell123@gmail.com>
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