[JS, Frontend] Add test with extension member in external interface

js/js.translator/testData/box/extensionFunction/inExternalInterface.kt

@@ -0,0 +1,9 @@
+// !LANGUAGE: +JsEnableExtensionFunctionInExternals
+
+external interface Foo {
+    fun foo(): String.() -> Unit
+}
+
+fun box(): String {
+    return "OK"
+}