Fix transformable label todo

[mo1ein]

Overview: What does this pull request change?

Motivation and Explanation: Why and how do your changes improve the library?

This PR improves type safety and code quality by:

• Creating a proper TransformableLabel class to handle transformable labels with clear type annotations
• Removing type ignore comments by adding proper type hints
• Improving the constructor to handle both MathTex and string transformation names
• Cleaning up kwargs management by separating MathTex constructor arguments
• Updating type annotations for the transformable_labels list
• Fixing the get_transformable_label_movement method to use the new attribute names

These changes resolve the TODO comment about unclear types in the add_transformable_label function and make the code more maintainable and type-safe.

Links to added or changed documentation pages

Further Information and Comments

The changes maintain backward compatibility while improving type safety and code clarity.

Reviewer Checklist

• The PR title is descriptive enough for the changelog, and the PR is labeled correctly
• If applicable: newly added non-private functions and classes have a docstring including a short summary and a PARAMETERS section
• If applicable: newly added functions and classes are tested