Adds macOS support for `flutter create`:
- Currently it is behind a hidden flag.
- Adds a TargetPlatform workaround to lib/main.dart in the standard app template when enabled.
- Supports `app` and `plugin`; `module` support doesn't yet exist for macOS in general.
This will eliminate the need to use FDE's examples as templates on macOS. The templates are based on the current state of FDE's examples, with templating support added (and with adoption of the new application delegate in the app, which hadn't been done yet in FDE, eliminating some boilerplate from the template).
Fixes#30703
This renames the "module" template to the "application" template, and makes "application" the default. The existing "app" template is now deprecated.
flutter create also now recognizes the type of project in an existing directory, and is able to recreate it without having the template type explicitly specified (although you can still do that). It does this now by first looking in the .metadata file for the new project_type field, and if it doesn't find that, then it looks at the directory structure. Also, the .metadata file is now overwritten even on an existing directory so that 1) the project_type can be added to legacy projects, and 2) the version of Flutter that updated the project last is updated.
I also cleaned up a bunch of things in create_test.dart, added many more tests, and added an example test to the test/ directory in the generated output of the application template.
Fixes#22530Fixes#22344
