export menu button

wax-prosemirror-components/index.js

 export { default as Overlay } from './src/components/Overlay';
 export { default as Button } from './src/components/Button';
+export { default as MenuButton } from './src/ui/buttons/MenuButton';
 export { default as icons } from './src/icons/icons';
 export { default as TableDropDown } from './src/components/tables/TableDropDown';
 export { default as ImageUpload } from './src/components/images/ImageUpload';