Make video file extension case insensitive

[sfmig]

Right now, we look for video files in the input directories with the extension provided, but the extension is case sensitive.

This PR:

• modifies the frame extraction script to search for video files with the provided extension, ignoring its case.
• includes a simple test (using two very short clips)
• makes "bboxes labelling" into a Python module (to be able to test its functions)
• adds a tests github actions workflow

Sorry if it feels like a big PR! But the large number of files is mostly because:

• I renamed "bboxes labelling" to "bboxes_labelling" to be able to use it as a module, and
• I added a pyproject.toml file, which has slightly different settings for ruff / black than what we had before, so a few files have been formatted.

So hopefully won't be too tedious to review!

The main content modifications are in:

• bboxes_labelling/extract_frames_to_label_w_sleap.py
• tests/test_frame_extraction.py