Support attaching to existing process

The next version of Dart Code will support attaching to an existing VM process (by pasting in an Observatory Uri). It doesn't support all the Flutter functionality (since we don't have a `flutter run --machine` instance to talk to - I've opened https://github.com/Dart-Code/Dart-Code/issues/905 to figure out what's missing and how we can fix that) but @mit-mit thinks IntelliJ might also not have this so there should be an issue open for it (I can't see an existing one).