dev/sg: migrate check.Runner to lib/group, add concurrency limit on checks
Created by: bobheadxi
Was trying lib/group
out while working on a blog post about generics for the content team. Limiting concurrency also seems important since I saw a PR doing something similar manually for sg start
(https://github.com/sourcegraph/sourcegraph/pull/35927). Hopefully I'm holding this right
Probably best reviewed by just reading runAllCategoryChecks
without the diff
Test plan
Tests pass, go run ./dev/sg