Add Priority Class

[PresleyChew]

Closes #45

Added Priority Class under Person Package to represent Priority Object which will be under Person Object later on

Let me know if it looks good! 🫶

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 81.81818% with 2 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
...main/java/seedu/address/model/person/Priority.java	81.81%	2 Missing :warning:

Files with missing lines	Coverage Δ	Complexity Δ
...main/java/seedu/address/model/person/Priority.java	81.81% <81.81%> (ø)	7.00 <7.00> (?)

[PresleyChew]

Closes #66