Embroider: Watch for Changes

[gossi]

With duo-pipeline support for classic and embroider pipeline, unfortunately themes are no longer watched for changes in the embroider build pipeline.

The reason being: Although embroider is at work (and webpack), broccoli is still doing the file watching and whenever a change is detected, the embroider pipeline is run again.

Currently, embroider is some sort of a "broccoli plugin". This will change in the future, by making broccoli the plugin of embroider.

Until then to support theme development in embroider mode is to watch theme source files (and then the embroider/webpack build pipleine will take care)