FR: jj untrack --local, jj untrack --ignore

[lf-]

Is your feature request related to a problem? Please describe.
I want to be able to easily untrack a file and have it stick without having to manually edit a gitignore/.git/info/exclude file. This could be achieved by automatically editing the relevant gitignore file (probably the nearest one to the file for --ignore?) to add the file, and for --local, .git/info/exclude.

Describe the solution you'd like
jj untrack could be taught how to make untracking sticky, and print a warning if the file will get immediately tracked again suggesting the command that sticks the untracking.

Describe alternatives you've considered

Status quo: have to open an editor/type another command to do the untracking operation.
Reduce the necessity for untracking things (see #323).

Additional context
Add any other context or screenshots about the feature request here.