Add editor services

RPCs:
- navigateToCode
- getActiveLocation

Events:
- activeLocationChanged
- themeChanged

https://github.com/dart-lang/sdk/blob/bc7e4d9fb583c9edeb90393f65989767e865e56f/pkg/dtd_impl/dtd_common_services_editor.md?plain=1#L4

These things should probably be moved to the Dart plugin eventually