Initialize diagnostics when opening a document

[Philipp-M]

I've noticed, that diagnostics that are sent from the language server aren't propagated when opening a document (via helix_view::Document::open). So that the language server needs to send a PublishDiagnostics again to populate the document with diagnostics. This can take some time (noticed with rust-analyzer), when opening a file via the workspace diagnostic picker (if it is sent at all).

This PR should ensure that diagnostics that are already in the editor instance are propagated when opening a document.
This has the additional advantage, that diagnostics can be rendered directly in the workspace diagnostic picker, when the file/buffer isn't open yet.

[Philipp-M]

Highlights in workspace diagnostics picker didn't work anymore with the newest changes, the last commit should fix that

[Philipp-M]

Since there were quite some merge conflicts to resolve, I tried again with reusing Editor::doc_diagnostics (by not acting directly on the editor reference, but instead the registry and diagnostics), so in that form it probably improves it.

Can you check again @pascalkuthe?

[pascalkuthe]

This looks pretty good to me now! Mostly just some style nits. I will test this locally for a few days.

One thing that is still missing is that you need to repopulate diagnostics on config reload in case the diagnostics feature was enabled/disabled for some LS.

[Philipp-M]

One thing that is still missing is that you need to repopulate diagnostics on config reload in case the diagnostics feature was enabled/disabled for some LS.

I think I have covered this now everywhere where the language configuration can be changed, let me know if I missed something (or was to thorough).