Move diagnostic calculation outside of the critical section

[artempyanykh]

Right now, diagnostic calculation is added as a hook and happens inside a critical section: https://github.com/artempyanykh/marksman/blob/main/Marksman/Server.fs#L216; only the actual public of the updated diagnostic happens asynchronously. This means that diagnostic recalculation happens on every keystroke/state_update which is not cool and can lead to timeouts (albeit only on really huge workspaces with hundreds or thousands of md files).

The good news that it should be pretty easy to move diag recalc off of the critical section and make it happen async.