Detect plural rules as part of the message

[PiotrRogulski]

Flutter's arb messages can contain plural ICU rules as part of the string, not just the entire message.

For example, this message: "hello_world": "I have {count, plural, one {an apple} other {{count} apples}}" correctly generates the following method:

@override
String hello_world(num count) {
  String _temp0 = intl.Intl.pluralLogic(
    count,
    locale: localeName,
    other: '$count apples',
    one: 'an apple',
  );
  return 'I have $_temp0';
}