Provide an option to limit whether multiple styles can be added

Problem/Motivation

Content editors can be confused by the fact that block text in the WYSIWYG editor can have multiple styles applied, even though those styles can conflict and result in confusing output.

Proposed resolution

Provide an option on the ckeditor5 style plugin that disallows multiple block styles from being added to a single block.

Remaining tasks

Provide an option on the ckeditor5 style plugin that disallows multiple block styles from being added to a single block.

User interface changes

The ckeditor5 style plugin has an option that disallows multiple block styles from being added to a single block.

Introduced terminology

n/a

API changes

TBD

Data model changes

n/a

Release notes snippet

TBD