Implement sort_by_priority and wire it into list --sort

[jithurx]

sort_by_priority() in src/tasklite.py is a stub. Implement it so high-priority tasks come first, then normal, then low, and call it from list_tasks() when a --sort flag is passed.

[riya007zak-eng]

Mariya

[GKMsm701]

i will do

[Aparna-932]

fix