Adds initial support for `flutter create` of apps and plugins. This is derived from the current FDE example app and sample plugin, with a few changes:
- Added template values where it makes sense.
- Moved some likely-to-change values into separate files for now, to simplify the delete/recreate cycle that will be necessary until it's stable.
- Added some minor Makefile flag handling improvements

Since the APIs/tooling/template aren't stable yet, the app template includes a version marker, which will be updated each time there's a breaking change. The build now checks that the template version matches the version known by that version of the tool, and gives a specific error message when there's a mismatch, which improves over the current breaking change experience of hitting whatever build failure the breaking change causes and having to figure out that the problem is that the runner is out of date. It also adds a warning to the `create` output about the fact that it won't be stable.