Implement Highlight feature

[jodator]

A feature for highlight a text by defined set of "highlighters". At first it looks very similar to font background but this feature would most probably have different UI/UX then a font background.

The main difference here is that Highlight feature is mostly inspired by highlighting a text on paper with a marker pen.

There's also a described use-case by @elglingo: #631 (comment) and some insights on this from @scofalik: #631 (comment).

[jodator]

And also related talk in Editor Recommendations repo: ckeditor/editor-recommendations#63.

[jodator]

As pointed out by @commander in editor recommendations. The element used to mark stuff in content is called mark.

[jodator]

@oleq & @fredck I've created an issue in https://github.com/ckeditor/ckeditor5-highlight/issues/1 that gathers all talks about this feature. Please add your comments if needed.

[Reinmar]

Further works are conducted in the https://github.com/ckeditor/ckeditor5-highlight repo.