Logic to interact with the `Editor` DTD service should be pulled into a standalone package and published

The [classes and logic for interacting with the `Editor` DTD service](https://github.com/flutter/devtools/tree/master/packages/devtools_app/lib/src/shared/editor) would also be useful for other DTD clients wishing to interact with IDEs (e.g., the widget previewer).

We should consider pulling this DevTools specific logic into a dedicated `dtd_editor_service` package that's published and provides an extension on `DartToolingDaemon` for interacting with the `Editor` service.