move untested extensions to the archive folder (see comment)

anyone wishing to test and confirm it should go back to the project can submit an issue (or better propose a PR).

archive/untested-extensions/ext-mathjax/locale/zh-CN.js

@@ -0,0 +1,8 @@
+export default {
+  name: '数学',
+  buttons: [
+    {
+      title: '添加数学计算'
+    }
+  ]
+};