feat(exports): export YOAST instead of adding to window

[mihaildu]

I think it's better to export the variable instead of adding it to window as a global, for example to avoid name conflicts, only importing it where you need it etc.

This will allow the users to import the YOAST object like this

import YOAST from 'netlify-cms-yoast-seo/dist/main.js';

Instead of doing this

import 'netlify-cms-yoast-seo/dist/main.js';