An integer has sequential digits if and only if each digit in the number is one more than the previous digit.
Return a sorted list of all the integers in the range
[low, high] inclusive that have sequential digits.
Input: low = 100, high = 300 Output: [123,234]
Input: low = 1000, high = 13000 Output: [1234,2345,3456,4567,5678,6789,12345]
10 <= low <= high <= 109