kt: add «125. Valid Palindrome»

URL:	https://leetcode.com/problems/valid-palindrome/
Signed-off-by: Matej Focko <me@mfocko.xyz>

kt/valid-palindrome.kt

@@ -0,0 +1,13 @@
+class Solution {
+    fun isPalindrome(s: String): Boolean =
+        s.toCharArray()
+            .map { c -> c.toLowerCase() }
+            .filter { c -> c.isLetterOrDigit() }
+            .toCharArray()
+            .let { s ->
+                s.indices
+                    .map { l -> l to s.size - l - 1 }
+                    .takeWhile { (l, r) -> l < r }
+                    .all { (l, r) -> s[l] == s[r] }
+            }
+}