Support textDocument/onTypeFormatting

[xzbdmw]

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• This is not a bug report or a question (Please use issues for those).
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Feature description

https://rust-analyzer.github.io/book/features.html#on-typing-assists

[mrcjkb]

Since this isn't an off-spec feature that is specific to rust-analyzer, this should be added to neovim and/or implemented as a separate plugin.

Closing as a duplicate of neovim/neovim#21191.

There's also this plugin (which I haven't tried myself).