Implement support for non-kts scripts in gradle

#KT-26784 fixed

libraries/tools/kotlin-gradle-plugin-api/src/main/kotlin/org/jetbrains/kotlin/gradle/plugin/KotlinSourceSet.kt

@@ -39,4 +39,6 @@ interface KotlinSourceSet : Named, HasKotlinDependencies {
         const val COMMON_MAIN_SOURCE_SET_NAME = "commonMain"
         const val COMMON_TEST_SOURCE_SET_NAME = "commonTest"
     }
+
+    val customSourceFilesExtensions: Iterable<String> // lazy iterable expected
 }