Add watch capability to any tool

[evanweible-wf]

Theoretically, it could be useful to be able to enable a "watcher" for any ddev target so that it automatically re-runs on filesystem changes. This could probably be implemented as a separate ddev watch target or as a common --watch mode on each individual target, and we could use package:watcher to accomplish this.

However, I'm not sure how useful this mode would be in practice. Consider:

• ddev analyze - IDEs already support live analysis results
• ddev format - IDEs support format-on-save and we are considering creating plugins for VSCode/IntelliJ that support custom formatters (ddev format or over_react_format)
• ddev test - this is the one that would probably benefit the most from a watcher, especially when used in smaller projects with fast re-build times or when used in tandem with the ddev test <path/to/test.dart> usage that automatically filters the build
• ddev serve` - already has a watcher builtin