* Building shared C source code as part of the native build for platforms Android, iOS, Linux desktop, MacOS desktop, and Windows desktop.
* Sample code doing a synchronous FFI call.
* Sample code doing a long running synchronous FFI call on a helper isolate.
* Use of `package:ffigen` to generate the bindings.
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
