Docs: Linked our example converter for highlight to font migration.

Other: Added font color and font background color features and removed the highlight feature. Closes ckeditor/ckeditor5#5886.

MAJOR BREAKING CHANGE: The document editor build does not contain the highlight feature anymore. It was replaced with the font color and font background color features. You should install the highlight feature back in case you expect that the highlight feature was already used by the users of the editor. Otherwise, the markup created in the past by that feature will be filtered out the next time it is loaded to the editor.

Tests: Fixed tests leaking editor instances / DOM elements. See ckeditor/ckeditor5#6002.

Docs: Error messages in manual tests and a sample should be more verbose should the editor fail to initialize (see ckeditor/ckeditor5#1803).

Other: Replaced UglifyJS with Terser. See ckeditor/ckeditor5#1353.