LSP needs to handle non existing files

[nojaf]

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.