feat: add purpose to storyboard entries and export segments to .skyc

[volfpeter]

Changes:

• Ruff commit hook upgrade
• Some typing improvements
• Some refactoring (e.g. rth, storyboard)
• Add a purpose enum property to StoryboardEntry
• Calculate show segments based on StoryboardEntry.purpose and make this info available for export
• get_storyboard() now only accepts keyword only arguments (with_context only supports context in keyword args)

[ntamas]

Thanks!