kt: add «2579. Count Total Number of Colored Cells»

URL:	https://leetcode.com/problems/count-total-number-of-colored-cells/
Signed-off-by: Matej Focko <me@mfocko.xyz>

kt/count-total-number-of-colored-cells.kt

@@ -0,0 +1,3 @@
+class Solution {
+    fun coloredCells(n: Int): Long = 1L + 2L * n * (n - 1)
+}