Add inputs-inferred yaml option

[skylon07]

Closes #6

This PR adds an option to automatically discover input files instead of manually mapping each input file to its output destinations. This is opt-in behavior that does not change/break existing configurations. This PR also includes some tests and an update to the README.

I figured a great package such as this one deserved a nice quality-of-life upgrade ;)

[skylon07]

For anyone who wants to use these changes now, you can depend on my github branch:
(this only works when depending on pigeon <24.0.0)

git:
  url: https://github.com/skylon07/pigeon_build_runner_infer_inputs.git
  ref: dependable
  path: packages/pigeon_build_runner

If you want to use pigeon 24.0.0 or later (aka include the fixes from #5), use this branch instead:

git:
  url: https://github.com/skylon07/pigeon_build_runner_infer_inputs.git
  ref: dependable-updated
  path: packages/pigeon_build_runner