Add running command status

[mfirhas]

Issue: #11433

[the-mikedavis]

About the use-case mentioned in #11433: :sh is meant for simple one-off commands (like ls or simple git commands like git rev-parse HEAD and isn't meant to be used to trigger builds or tests. Larger tasks like that should live in a separate terminal pane and maybe #1976 in the long run.

cargo run ... is a good example of why :sh is meant for simple commands: if you have a compilation error, the popup with the error goes away when you dismiss it and there's no way to read it back.

For short-lived commands this change worsens the UI a bit since the "Running command ..." message pops up and then is immediately replaced with "Command run" so the UI 'flashes'. For long running jobs I'd like to see a UI more like #6436 (which we might be able to reuse for shell commands too). I will close this out since I don't think we will push this change forward, but I'd be interested in experimenting with a UI for long-running jobs.