Ignore stray textDocument/didSave requests

[mmyrte]

Zed chose to interpret the LSP spec differently from VS Code, sending didSave to any file, not caring what the languageId is - see ocsmit/zed-r#6 - which leads to languagserver trying to parse and lint all over the place. (edit: found #681 also relates to this issue).

Dirk Bäumer tentatively specifies that a server should not respond to a request if the file was not previously opened by it, see microsoft/language-server-protocol#2110, see hudson-trading/slang-server#237 for a sample implementation.

I'm proposing to simply ignore documents that are not yet in the workspace, which fixes the problems on my end. I'm afraid I don't know enough about this package or the protocol to assess whether this is acceptable; I also don't have the time to go into this much further

I am happy to amend this PR, if needed: we could specifically retrieve the document associated with a URI to check for the is_open flag, but that leaves open the question of whether/how to ignore files not yet in the workspace.

[JonGretar]

Thank you.

[rgbkrk]

Thank you! I'm not a maintainer here obviously but I would love to have this fix.

[renkun-ken]

LGTM thanks!

[renkun-ken]

Looks like the change breaks most test cases where each test code relies on did_save to open and parse the supplied document.

Testing should be fixed