Add CKEditor 5 specific stylesheet

This merge request has not been tested and does not yet include the built assets.

I agree that this needs to utilize @use instead of @import, also this should be on the 5.x branch.

@jldust / @pixelwhip - I opened a new branch off of 5.x and updated the WYSIWYG file to use `@use` statements. I didn't add the class wrapper, but do we really need it? As long as the styles that are being defined for the front end are also being loaded into CKEditor, we shouldn't need the wrapper?

@pixelwhip explained to me that CKEditor5 is now not loaded as an iFrame, versus how CKEditor4 is being loaded, so if we don't load our styles inside of the CKEditor wrapper class, they will be applied to the admin theme as a whole.

@jldust / @pixelwhip - I've gone the route of using mixins for necessary theming that is needed for CKEditor. I've added comments everywhere that I am doing this to be clear on what is going on.

Thanks for these changes, I've merged this in and it will be included in the next tagged release.

This has been included in the 5.x.2-alpha1 releases.

jldust → closed merge request !58