Prevent concurrent RW access to the same file

[pr2502]

This is not strictly our domain but we could add a very simple fix to it, maybe.

Helix has currently no mechanism for preventing multiple concurrent RW opens of the same file helix-editor/helix#4257 which has already caused me real data loss by saving an old buffer from a forgotten window.

Once open document tracking per client is implemented (to resolve #44) ra-multiplex could solve this helix issue by synthesizing a LSP error when it receives a DidOpenTextDocument for a file that is opened by another active client. It seems very simple so I want to try it, but we'll have to see how well that works.