Toggle plugin

[zbeyens]

A toggle that hides the children. This needs some work in Slate to support invisible children.

[Liboul]

Would love this!

I researched this briefly, here are my 2 cents:

1. There seems to have been one guy who tried to do this and succeeded without the need for core changes in Slate
   Hidden nodes (blocks and inlines) ianstormtaylor/slate#1211. That was years ago though.
2. The native HTML element <details> has this toggle behaviour: https://developer.mozilla.org/en-US/docs/Web/HTML/Element/details. I've seen a CKEditor plugin leveraging this

[Liboul]

@zbeyens I just submitted a PR that implements toggles: #2895