style: align anywidget sorting properties with python conventions

[shuoweil]

This PR standardizes the sorting traitlets in TableWidget to strictly use snake_case, adhering directly to the Python style guide for instance variables rather than relying on workarounds to conform to JavaScript conventions.

1. Removed sync_key dictionary configurations from sort_columns and sort_ascending traitlets in anywidget.py
2. Updated the frontend table_widget.js to dispatch state using the individual sort_columns and sort_ascending event properties, instead of a combined sort_context.
3. Rewrote all assertions test against the new separated list property structures.

Fixes #<487761031> 🦕

[shuoweil]

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