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/repl/analyzeErrors.repl

@@ -1,6 +1,6 @@
 >>> fun foo() = 765
 null
 >>> foo(1)
-ERROR: /line2.ktscript: (1, 5) Too many arguments for internal final fun foo(): kotlin.Int defined in <script>
+ERROR: /line2.kts: (1, 5) Too many arguments for internal final fun foo(): kotlin.Int defined in <script>
 >>> foo()
 765

compiler/testData/repl/syntaxErrors.repl

@@ -1,6 +1,6 @@
 >>> )(
-ERROR: /line1.ktscript: (1, 1) Expecting an element
-ERROR: /line1.ktscript: (1, 3) Expecting an expression
+ERROR: /line1.kts: (1, 1) Expecting an element
+ERROR: /line1.kts: (1, 3) Expecting an expression
 >>> fun foo() = 98
 null
 >>> foo()