Use inert polyfill (#12209)

* Inserting wicg-inert in project

* Import wicg-inert in moda_root component

* Update yarn.lock

* Solving code style problems

* Removing package-lock

app/javascript/mastodon/components/modal_root.js

@@ -1,5 +1,6 @@
 import React from 'react';
 import PropTypes from 'prop-types';
+import 'wicg-inert';
 
 export default class ModalRoot extends React.PureComponent {