[ffigen] Migrate `flutter create` template to Dart API

[goderbauer]

To prepare for the downturn of the YAML API.

[dcharkes]

Do you mean flutter create --template=package_ffi to dart create --template=package_ffi? Or do you want to migrate the readme of ffigen to a Dart console or cli app? (If the former, it's not really an FFIgen-specific issue.)

[goderbauer]

When you currently execute flutter create --template=package_ffi foobar it generates an app that configures ffigen via a YAML file. We should change that to instead use a Dart script to configure ffigen via its Dart API and delete the YAML file from that template.

(cc @liamappelbe that's what you meant as well, right)

[dcharkes]

Ah, migrate to Dart API. Not to dart. 👍

We should probably first release 20.0 stable.