refactor registering fixes for each rule #285

void main() {
  final rule = DoubleLiteralFormatRule();
  registry.registerLintRule(rule);
  registry.registerFixForRule(rule.code, DoubleLiteralFormatFix.new);
}

void main() {
  final rule = DoubleLiteralFormatRule();
  registry.registerLintRule(rule);
  for (final MapEntry(key: code, value: fixFn) in rule.fixesForCodes) {
    registry.registerFixForRule(code, fixFn);
  }
}