kt: add «3174. Clear Digits»

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

kt/clear-digits.kt

@@ -0,0 +1,11 @@
+class Solution {
+    fun clearDigits(s: String): String =
+        s.fold(mutableListOf<Char>()) { chars, c ->
+            when {
+                c.isDigit() -> chars.removeLast()
+                else -> chars.add(c)
+            }
+
+            chars
+        }.joinToString(separator = "")
+}