[Feature Request] Add class to checklist items if checked

[ttytm]

Add a class if items in a checklist are checked to allow further options with styling and visualizing in the editor view. In the preview-view the items get a "is-checked" class.

Thoughts on a solution: With the current layout structure its probably good to add the class to the <pre> parent tag if the child <span class="... .cm-formatting-task ..." has data-task="x" to target the whole line. Because when creating themes only with css it's not possible to style from children to parents backwards. But there are many ways to get that solved.

Thanks for you work already!

Kind regards