Add filewatcher/v2

Being the first lib implemented in baseplate.go and long predates
generics, the API of filewatcher lacks type safety and can cause other
annoyances.

Add filewatcher/v2 that provides:

* Generics/type safety
* Use io.Closer over Stop()
* Use With- options over a Config struct
* Use slog at error level over log wrapper

And change original filewatcher lib to a thin wrapper of v2.

This change intentionally does not change any of its users (experiments,
secrets, etc.), as a proof that we did not break v0 API. The next change
will switch them to use v2.