pubspec_parse: Added support for false_secrets

[leoafarias]

Description:

This PR introduces support for false_secrets in the pubspec.yaml file. The false_secrets field allows users to specify files that should be ignored when checking for secret leaks before publishing. This helps avoid false positives during the publishing process.

The flutter_favorite_pubspecs.json file contains the pubspec configurations for Flutter Favorite packages from pub.dev. Its inclusion in the test suite is intended to expand the test coverage footprint, specifically to catch any potential parsing issues with the Pubspec class.

Changes:

• Added false_secrets field to Pubspec class in lib/src/pubspec.dart.
• Updated generated code in lib/src/pubspec.g.dart to handle false_secrets.
• Added tests to validate false_secrets field parsing and behavior in both Map and Yaml formats.
• Added test fixtures flutter_favorite_pubspecs.json and sample_pubspec.yaml to cover various cases.

Checklist:

• I’ve reviewed the contributor guide and applied the relevant portions to this PR.
• Tests have been added for the new feature.