Export Briefer Notebooks and Support for Versioning

### **Description:**

Currently, Briefer does not allow users to load .ipynb files. It would be very useful to implement a feature that not only allows loading .ipynb files but also exporting Briefer notebook files. This would facilitate proper version control, making it easier for users to track and manage their Briefer notebooks in repositories like Git.

### **Feature Request:**

Add functionality to load .ipynb files into Briefer.
Add functionality to export Briefer notebooks to a file format that can be versioned (e.g., .briefer or .ipynb).
Ensure compatibility with version control systems (e.g., Git) to allow users to track changes to Briefer notebooks effectively.

### **Rationale:**

Enabling both import and export of notebooks would enhance flexibility, especially for users who work with Jupyter notebooks.
The ability to export and version notebooks is essential for maintaining workflow integrity and collaborative work environments.
Thank you for considering this feature!

Start by locating the notebook loading and export entry points and checking how Briefer notebook files are represented. Define the import and export formats, then verify that notebooks round-trip correctly and produce files whose changes can be tracked effectively with Git.

Written by the indexing model from the issue text.