compute: add regexp output command

Created by: rvantonder

Stacked on https://github.com/sourcegraph/sourcegraph/pull/26813.

As in title. You can do like output(func (\w+) -> $1)

⚠️ Note to self: the left hand side is interpreted according to our search regex convention, which is func.*(\w+). I don't like this hidden fuzzy convention for this command, and will make it conform to strict regex in a follow up PR.