Add assign/update Priority feature

[PresleyChew]

---

Feature: Add/Update Priority for Patients

• Purpose: Assign/Update a priority level to patients to manage urgent cases.
• Command format: updatePriority [i/NRIC] [p/PRIORITY]
• Example commands:
  • updatePriority i/T0123459I p/HIGH
• Acceptable values:
  • NRIC:
    • Must be 9 characters, with first and last characters being alphabets
    • Follows the 9-character Singapore NRIC format for uniqueness.
    • Case-insensitive (e.g. ‘TXXXXXXXB’ is the same as ‘tXXXXXXXb’) for consistency.
    • Priority: Allowed values are LOW,MEDIUM,HIGH.
      • Predefined valeus (LOW,MEDIUM,HIGH) for clear urgency classification.
• Error message if invalid:
  • "Priority value must be 'LOW', 'MEDIUM', or 'HIGH'."
  • “Patient does not exist in contact list”
  • “You can only assign priority to patients”
• Outputs:
  • Success: "Priority 'HIGH' assigned to John Doe."
  • Failure: "Invalid priority value."
• Duplicate handling: Updates existing priority rather than allowing duplicates.

---