Fix multi-cursor copy/paste on linux (#13523)

The clipboard library we use for X11 doesn't yet support multiple
formats on the clipboard, so for now we just store this in memory for
the current zed process, as we do for Wayland.

Fixes: #11971

Release Notes:

- N/A

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Co-authored-by: Mikayla Maki <mikayla@zed.dev>