dev: add go-mockgen
Created by: LawnGnome
This uses the same pattern as the main Sourcegraph repo: something like this will result in a mock file being generated:
//go:generate ../../dev/mockgen.go PARAMETERS
I'm just opening this as a draft and closing it so we can find it later if we need it — the need I had turned out to not merit this complexity, but I suspect one of us will want go-mockgen at some point in src-cli.