Support platform specific default-flavor

### Use case

Currently the pubspec.yaml's default-flavor assumes that the flavor name exists in all platforms that support flavor, which might be a wrong assumption.

For example in the app I am working on, it only has flavors for android and not for iOS, which works for us. But we also want default-flavor, but we can't have it since it assume that flavor exists on iOS and `flutter run` fails.

Also in iOS we have schemes and not flavors. Schemes are generally in PascalCase and not in snake_case or camelCase. So someone might want a pascal case scheme name for iOS and camelCase for android.

### Proposal

Allow default-flavor-$platformName as a parameter if it is present else use default-flavor, this way if someone only has flavor's on android, they can just do:
default-flavor-android: $flavorName

Or do:
default-flavor-android: $flavorName1
default-flavor-iOS: $flavorName2