Adds timer feature with pause/resume and unanswered filter

[chyke007]

Adds timer feature with

• Timer Support in HH:MM:SS
• Displays total time taken after quiz completion
• Completes quiz and submits once time runs out
• Support Pause/Resume Timer
• Shows unanswered questions
• Filter unanswered questions

Sample image:

[wingkwong]

• prefer to put the explanation under the tags. now it will break the layout.

Expected:

• pause button not quite aligned with Time Remaining / Question

• we need to add validation for timer and allowPauseTimer in Quiz.jsx. Check if it is valid number and boolean respectively.