Create and add a plugin for embedding playground iframes in docs

[jonoaldersonwp]

The docs team would like to be able to embed playground instances into content. We should create a simple plugin ("Playground Embedder") to facilitate this.

This should be relatively simple, but has a few moving parts:

• Someone needs to write a/the plugin
• The playground should be loaded via a shortcode (as content is managed in GitHub / sync processes, so a block-based approach isn't suitable)
• Playground content is delivered as an iframe, but we also need to consider;
  • The shortcode should support providing parameters (details at ​https://wordpress.github.io/wordpress-playground/docs/query-api/) which passes additional parameters to the iframe URL.
  • The iframe should only be loaded on click/interaction; so we need an 'inactive' state (which should presumably come with some form of skeleton view + sizing)
    • This will require some simple JS (which should only be enqueued on pages where the shortcode is present) and CSS (for styling the inactive state, and for managing the h/w/etc of the active state)

The base playground URL is https://playground.wordpress.net/.

[joostdevalk]

Created a small plugin for this purpose: ​https://github.com/jdevalk/playground-embedder

Fixes https://meta.trac.wordpress.org/ticket/7040