This implements https://github.com/flutter/flutter/issues/132654#issuecomment-1738221257, namely:

Make `Chromium.close` more robust:

* Send `SIGTERM` and wait up to 5 seconds, if the process exited, great! Return from the function.
* If the process has not exited, then send a `SIGKILL`, which is a much firmer way to exit a process. Same as before, wait up to 5 seconds, if the process exited, great! Return from the function.
* If it still hasn't exited then give up trying to exit Chromium, just print a warning to the console and return from the function.

Bonus: a few nullability fixes and extra `-v` logging.

Fixes https://github.com/flutter/flutter/issues/132654