Same as `editor::RestartLanguageServer`, now there's an
`editor::StopLanguageServer` action that stops all language servers,
related to the currently opened editor.

Opening another singleton editor with the same language or changing
selections in a multi buffer will bring the servers back up.

Release Notes:

- Added a way to temporarily stop LSP servers

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Co-authored-by: Michael Sloan <mgsloan@gmail.com>