Rich status bar support

[lnicola]

See https://code.visualstudio.com/updates/v1_59#_rich-status-bar-hover.

We could have actions for status, memory usage, maybe even cancelling a running cargo check (an often-requested feature).

[Milo123459]

Hey 👋
I'd be happy to take this up. Here are some ideas I've had to display:
Display an X button next to cargo check: X cargo check, and that cancels it.
Show memory usage, and a toggle (which I believe has been added to rust-analyzer) to only show errors, not warnings.
If the last one hasn't been implemented, I'd be fine to implement that one as well.

[Zingam]

How about run, debug, etc. stuff?