Rename Extension annotation to ExtensionFunctionType

The old one is kept temporarily and will be removed later

idea/idea-completion/testData/basic/multifile/ExcludedJavaClass/somePackage/ExctraClass.java

@@ -1,6 +1,6 @@
 package somePackage;
 
-// Such strange name here needed to make completion prefix not intersecting with kotlin.Extension
+// Such strange name here needed to make completion prefix not intersecting with kotlin.ExtensionFunctionType
 public class ExctraClass {
 
-}
+}