LSP needs to handle non existing files

Not sure what we should do but I had the following:

- I opened a new `.res` file on a specific branch.
- I switched to the main branch (where the new file doesn't exist)
- My editor still holds the removed file.

This now leads to a crash in the LSP server.

We probably need to clean up the internal state and not try to do provide any lsp responses for that file.