Implement `queueFront(ops)` where each op is `{ type: 'enq', val }` or `{ type: 'deq' }`. Apply them and return the value at the front, or `null` if empty.
+ 1 hidden test run on Submit.
Replaying the operations on an array models a queue; the front is the first element or null when empty.
Run your code to see results.