Design a function createCounter that takes an optional starting number. The function should return another function which, when called for the first time, returns the starting number (or 0 if no starting number is given). Each subsequent call should return the next integer in sequence, increasing by 1 from the previous value.

Example:

const countUp = createCounter();
countUp(); // 0
countUp(); // 1
countUp(); // 2

const countFromFive = createCounter(5);
countFromFive(); // 5
countFromFive(); // 6
countFromFive(); // 7