Add highlighting to simulate wysiwyg inside the editor

This adds a highlighter to the editor which will mimic (to some extent) a wysiwyg editor. More could and might be done depending on feedbacks.

Settings to configure the size of style boundaries have been added to make them more or less visible.

The highlighter will show the boundaries in all cases when the cursor is surrounded/right next to a boundary.

Demo:

This also introduce a new Demo note.