[Davidvisualstudio007]

Closes #323

This adds a save-chapters command that saves the detected scenes as chapters in FFmetadata format. You can then add them to a video with ffmpeg without re-encoding:

scenedetect -i video.mp4 detect-content save-chapters
ffmpeg -i video.mp4 -i video-Chapters.txt -map_metadata 1 -codec copy output.mp4

What I changed:

• Added write_scene_list_ffmetadata() to scenedetect.output. I based it on the EDL writer, so it works with a file path or an open file
• Added the save-chapters command with -f, -t and -o options, and a [save-chapters] section in the config. -t lets you change chapter names using $SCENE_NUMBER
• Special characters in titles get escaped so ffmpeg doesn't misread them
• Updated the docs, changelog and features list

For the timing I used _frame_timecode_seconds(), so it stays accurate at 23.976 and 29.97 and doesn't drift like the EDL issue in #593.

I added 22 tests in test_output.py and 2 in test_cli.py, including one that actually runs ffmpeg to add the chapters to a video and checks they're there. Ruff passes and the output and CLI tests all pass on the latest main. I also tried it on goldeneye.mp4 and the chapters showed up in VLC.

I left YouTube style chapters out for now. YouTube needs the first chapter at 00:00, at least 3 chapters, and each one at least 10 seconds long, so I wasn't sure how you'd want short scenes handled.