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

[bkonyi]

The classes and logic for interacting with the Editor DTD service 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.