Add `lets-run.config.ts` support

[paksa92]

It would be nice to be able to have a config to define paths for --on-path-exists and --watch. I have a monorepo at a client that is growing fast and has many packages depending on many other packages, and maintaining these paths in a package script is not ideal.

I will see if I can make some room in my schedule next week to implement this and create a PR, but if you're able to do it sooner that would be great.