FR: Allow exclusions in sparse checkouts

I'd like to be able to specify a few prefixes to exclude from my working copy.

Currently jj sparse set only allows positive patterns. To get what I want, I have to

• jj sparse set --clear
• jj sparse set --add everything except the patterns I want to exclude
• continually repeat that previous step as new paths are introduced

It would be a lot more convenient to just be able to do something like jj sparse set --remove someprefix, or add some negated prefixes via jj sparse set --edit and have it work.