Hi, this pull request adds support for `rubocop` language server. I
noticed that `ruby-lsp` LS is becoming more popular but it still lacks
diagnostics support in Zed. To cover that missing feature, it could be
good to use `rubocop` LS to show diagnostics alongside with the running
Ruby LSP.

Release Notes:

- N/A

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Co-authored-by: Marshall Bowers <elliott.codes@gmail.com>