Implement did_close() and per-file diagnostics

[DavisVaughan]

A step towards posit-dev/positron#1005

Filled in did_close(), which should be useful if we decide we want to regenerate diagnostics for all open files.

Also switched away from using a global variable for DIAGNOSTICS_VERSION altogether. I believe we can safely use the per-file version for this, which is thread safe due to it being inside a DashMap (aka mimicking a RwLock<HashMap>). This change allows multiple files to enqueue diagnostics at the same time (i.e. we no longer cancel file A's diagnostics if file B enqueue's theirs), which will also be necessary for regenerating diagnostics for all open files.

I originally thought I was having some kind of deadlock issue with this switch, but I think I may have just compiled a faulty version of ark on my end because the problem went away after a full recompile. It does make me slightly nervous though. I will keep an eye out for anything suspicious over the next few days.

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454bae0 contains the switch to using a per-file version check, a941b77 is some further tweaking of enqueue_diagnostics_impl() so that it no longer needs to be async and has this clear responsibility of Document in -> Diagnostics out

[kevinushey]

LGTM!