Add toggle to completely disable diagnostic rendering

[kirawi]

This has been a commonly asked feature and currently there isn't a way to completely disable them as far as I can tell (or it isn't obvious to me). It should be fairly straightforward from what I can tell from https://github.com/helix-editor/helix/blob/master/helix-term/src/ui/editor.rs

[NikitaRevenco]

If such command is added it shouldn't hide the diagnostic symbols, but rather only hide the inline diagnostics or the overlay diagnostics (default action)

the reason why, is because gutter symbols are not intrusive. I may want to disable diagnostics for some time to avoid them getting in the way, but still having the information that there are errors on those lines is useful