Improve playlist filenames

[alecigne]

The playlists directory contains files whose filename is in the [id].json format. That makes identification of a given playlist difficult. The goal of this dev would be to include the playlist title in the filename, but in downcase and cleaned from any undesired characters (whitespace, dashes, special characters, etc.).

• Hip-Hop -> [id]_hip_hop.json -- downcase, dash transformed into underscore
• Funk and Soul -> [id]_funk_and_soul.json -- downcase, whitespace transformed into underscore
• Funk & Soul -> [id]_funk_soul.json -- downcase, whitespace and "&" transformed into underscore, more than 1 underscore limited to 1 underscore
• Downtempo / Nu Jazz -> [id]_downtempo_nu_jazz -- downcase, whitespace and "/" transformed into underscore, more than 1 underscore limited to 1 underscore
• Drone Zone [Study / Relax / Meditate] -> `[id]_drone_zone_study_relax_meditate.json -- downcase, whitespace, "[", "]" and "/" transformed into underscore, more than 1 underscore limited to 1 underscore

Use jqwik to write a property-based test for this logic.

[alecigne]

Implemented in v0.4.0.