Stagehand integration for Dart project template creation (WEB-13936).

Defers to stagehand for template creation.  In case stagehand is not available (e.g., in the unlikely event the SDK is pre 1.6 and we can't run pub global activate) or an error occurs in querying it we fall back on pre-existing static templates.  The main entry point file and pubspec are opened post creation (like the other templates) and similarly, pub operations are run.

More context in associated issue here: https://youtrack.jetbrains.com/issue/WEB-13936