[SuperEditor] - Toggleable headers

[matthew-carroll]

Implement toggleable headers such that tapping an indicator next to the header collapses all content below the header, before another header of an equal level.

It should be possible to animate (or not) the collapse and expansion of the children.

The toggle indicator should have a sane default (probably including stylesheet properties), but should be fully configurable. The indicator should be able to animate (or not).

Then, do the same for list items. For list collapsing, please check for a reference app to see how it looks and behaves elsewhere.

After implementing this for headers and list items, look for any opportunities to generalize this behavior, or make it easier to create other collapsible things in the future.

Ideally, collapsing would be worked into the existing components for headers and list items. However, if it's discovered that this behavior is disruptive enough that new components should be created, that's OK, too.

Additional requirement

To facilitate certain client needs, it should also be possible to configure how many child nodes fall into the net of collapsing nodes. For example, if an app only wants to collapse the node immediately after it, it should be possible to configure that with these APIs.