Simplify WYSIWIG stylesheet approach

Problem/Motivation

Currently we rely on SCSS to wrap ckeditor 5 stylesheets in .ck-content. This is cumbersome as it requires wrapping base and element styles in a mixin, then immediately calling that mixin.

There are also a couple existing bugs related to this approach:

1. The wysiwyg stylesheet outputs unprefixed styles along with the prefixed styles. Which will cause rendering issues in admin themes.
2. :root selectors get prefixed with .ck-content which results in an invalid selector.

Solution

Install and configure postcss-prefixwrap