kt: add «1957. Delete Characters to Make Fancy String»

URL:	https://leetcode.com/problems/delete-characters-to-make-fancy-string/
Signed-off-by: Matej Focko <me@mfocko.xyz>

kt/delete-characters-to-make-fancy-string.kt

@@ -0,0 +1,7 @@
+class Solution {
+    fun makeFancyString(s: String): String =
+        s.windowedSequence(3, partialWindows = true)
+            .filter { it.length < 3 || it[0] != it[1] || it[1] != it[2] }
+            .map { it[0] }
+            .joinToString(separator = "")
+}