Add support and mode for workspace pull diagnostics

[jwortmann]

This adds support for workspace pull diagnostics and implements alternative 2 from #2224 (comment) by renaming the (unreleased) hide_non_project_diagnostics client setting.

If the renaming of the setting is suitable, then the next release of the LSP package should wait until it is merged, so that there would be no breaking change.

It's incomplete and work in progress at the moment, but feel free to comment on the ideas or push the branch in case someone wants to work on this.

The bad thing is that it cannot be tested, because afaik there is no server with support for diagnosticProvider.workspaceDiagnostics capability yet (?).

[jwortmann]

I found that the luau-lsp server for Lua has support for workspace pull diagnostics. And there is a bit of background info about their implementation here: JohnnyMorganz/luau-lsp#332

Unfortunately I don't know much Lua / Luau and therefore couldn't really set up a project to accurately test background diagnostics for inter-file dependencies.

[rchl]

I've merged the conflicting change but I'm fine with switching to this. I think, haven't looked at the changes and thought about it much yet.