feat(lsp): implement show document request (#8865)

* feat(lsp): implement show document request

Implement [window.showDocument](https://microsoft.github.io/language-server-protocol/specifications/lsp/3.17/specification/#window_showDocument)
LSP server-sent request.

This PR builds on top of helix-editor#5820,
moves the external-URL opening functionality into shared crate-level
function that returns a callback that is now used by both the
`open_file` command as well as the window.showDocument handler if
the URL is marked as external.

* add return

* use vertical split

* refactor

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Co-authored-by: Michael Davis <mcarsondavis@gmail.com>

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