ES6: Add support for findIndex in Arrays

The following discussion from https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/1629 should be addressed:

• @timzallmann started a discussion: (+4 comments)

  Do we have a Polyfill for .findIndex ?

Add Array.prototype.findIndex support;

Just add the following line to commons/polyfills.js (after the polyfill for /fn/array/find

import 'core-js/fn/array/find-index';