Feature summary:
Kartographer marker server should support "two-letter-icon" instead of forcing single-letter-icon only.
Currently, Kartographer Wikimedia markers only provide single-letter for letter icons.
However, the two-letters are also frequently used in many places' online/physical maps.
We should allow the letters to counted from single-letter to two-letter like Excel did: A, B, C, ..., X, Y, Z, AA, AB, AC, ... ZX, ZY, ZZ.