WEB: support watch all of web_ui folder instead of only web_ui/lib

### Use case

When run `felt build --watch` in this project I find some problems with this code https://github.com/flutter/flutter/blob/master/engine/src/flutter/lib/web_ui/dev/build.dart#L96, flutter only supports watch `web_ui/lib` folder change when developing locally. I hope there can support watching all of the web_ui folders like the `flutter_js` folder because I want to re-execute the js bundle build by esbuild when files in `flutter_js` changed

### Proposal

Change `final FilePath libPath = FilePath.fromWebUi('lib');` to ` final FilePath libPath = FilePath.fromWebUi('.');
`

If it can be approved, I can submit a pull request