Add listCollections method to Firestore class

[shtse8]

This pull request adds the listCollections method to the Firestore class. This method allows retrieving all root collections from Firestore, similar to the firestore.listCollections() function in the Node.js Firestore library.

Changes:

• Implemented listCollections method in lib/src/firestore/firestore.dart.
• The method retrieves all root collections from Firestore.

Testing:

• Please verify the functionality and run the provided tests.

References:

• Issue: Firestore is missing a function for reading root collections