feat(player): add `is_playing` helper

[spifory]

Summary

• Closes #60

Adds a Player.is_playing helper function by making use of https://github.com/ooliver1/mafic/issues/60#issuecomment-1497491105

Checklist

• [ ] If code changes were made then they have been tested.
• [x] I have run task lint to format code and my changes.
• [x] I have run task pyright and fixed the relevant issues.