[jnigen] watch mode

[dcharkes]

Package build_runner has a watch mode, to enable generating files on save.

If we introduce a hook/generate.dart it would likely have a batch mode, and a watch mode.

FFIgen currently only has a batch mode, we should consider starting to support a watch mode.

This will require first solving:

• [jnigen] Output dependencies #1009

[dcharkes]

(If we'd then hook/generate.dart scan the Dart files for // import com.example.foo.Foo.java grab those lines and put them in a generated jnigen config, and report both the Java files and the Dart files as dependencies, you'd add an import comment to a Dart file, wait a bit and then be able to call the Java function!)