Add dmypy watch command

[NiclasvanEyk]

Fixes #13033

This PR adds the dmypy watch command. It is basically the same as repeatedly running dmypy check FILES and then sleeping for X (defaults to 1) seconds. Here is a short video of it in action:

Screen.Recording.2025-04-27.at.20.50.36.mov

I wrote a new non-data-driven test that takes about 10s to complete on my local machine. I don't think an implementation in daemon.test would have been as readable, so I resorted to a classic unittest.TestCase.

Alternatives

This polling approach is quite simple and maybe a bit naive, but it works without introducing additional dependencies. For large projects, I assume that listening to filesystem events and only re-checking changed files (with some debouncing logic) is a better fit.

Since Python does not offer a builtin, cross-platform solution for listening to filesystem events, one would need to use third-party dependencies like watchdog to solve this, e.g. as an optional dependency. If it is installed, one could use a the more efficient approach, otherwise fall back to the simple polling approach. However, this is outside of the scope of this PR.

[github-actions]

According to mypy_primer, this change doesn't affect type check results on a corpus of open source code. ✅