- Removes `rootDirs` so all imports will be relative in the frontend
- Updates (and unifies) imports to be `../types/...` etc instead of
relying on "types" being in the rootDir for the frontend.
- Fixes one type that was being picked up from `lib` in the frontend.
- Adds a precommit hook to check in future
Paves the way to writing _unit_ tests for the frontend for the subset of
the frontend code we can import from `node` (which might be a lot of
it!)
