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/psi/script/ComplexScript.kts

@@ -0,0 +1,12 @@
+fun die(message: String) {
+    System.err.println(message)
+    System.exit(1)
+}
+
+if (args.length == 0) {
+    die("Need arguments")
+}
+
+for (arg in args) {
+    println(arg)
+}