feat(dart): add support fort dart generator

[jlabeit]

What the PR Includes

• Adds support for dart generators from mixpanel lexicon. Fixes https://linear.app/impargo/issue/DFS-1385/[lexigen]-dart-code-generation-support.

Example code that is generated:

import 'package:mixpanel_flutter/mixpanel_flutter.dart';

class Lexicon {
  static Mixpanel? mixpanel;

  static void _track(String eventName, {Map<String, dynamic>? properties}) {
    mixpanel?.track(eventName, properties: properties);
  }

  /// Clicked on save & continue while creating an order during the send flow.
  static void trackSaveContinueEvent({String? featureName, String? workflow}) =>
      _track('Save & continue', properties: <String, dynamic>{
        'Feature Name': featureName,
        'Workflow': workflow,
      });

}

Checklist

• Tests are added.
• Manual testing instructions are added.
• Configs are added/adapted if applicable.
• Issue is properly linked if applicable.

Manual Testing Instructions

1. To test the PR run the command below to generate a dart file.
2. Alternative you can checkout this PR that implements an example in the IMPARGO DriverApp.

Command to generate:

 npx ts-node src/index.ts  -u $USERNAME -s $KEY -p $PROJECT -t dart > lexicon.dart 

[KhaledSakr]

Looks good to me! Just one small suggestion.