perf(tactic/lint/frontend): run linters in parallel (#6293)
With this change it takes 5 minutes instead of 33 minutes to lint mathlib (on my machine...).
https://leanprover.zulipchat.com/#narrow/stream/113488-general/topic/linting.20time
Co-authored-by: Bryan Gin-ge Chen <bryangingechen@gmail.com>
