Remove wp-now from the main repository so it can live in a dedicated …

…space. See #473. (#477)

## What?

Removes `wp-now` from the main WordPress playground repository so it can
live in a dedicated space. This PR also removes VS Code extension –
merge WordPress/wordpress-playground#476 first.

As outlined in #473, WordPress Playground is re-focusing on [the
vision](WordPress/wordpress-playground#472)
and separating the framework from the applications.

Once the code removed by this PR is committed to a separate repo, a link
will be added to this PR.