Unify script file extensions to "kts" so that IDEA, compiler and tests agree when to parse as SCRIPT.

Include kt* files into formatter test generation, to test scripting formatting

compiler/testData/codegen/script/topLevelFunction.kts

@@ -0,0 +1,11 @@
+// expected: rv: 3628800
+
+fun factorial(n: Int): Int {
+    var product = 1
+    for (i in 1..n) {
+        product *= i
+    }
+    return product
+}
+
+factorial(10)