[List] support "expand and collapse" from List spec

[thw0rted]

Please describe the feature you would like to request.

The List spec includes an "Expand and collapse" control. This functions more or less like an Expansion Panel but with List Item styling: dividers are used instead of a surrounding border / different background color, and the expand/collapse button is placed according to the rules for secondary action buttons.

What is the use-case or motivation for this proposal?

Support full capabilities of List as described in the spec

Is there anything else we should know?

I put together a StackBlitz showing the current behavior when an expansion panel is placed in a list. The behavior is more or less as expected but the styling does not work as a list item, and just "fixing up" the current behavior is a lot to ask of consumers.

In my own project I built a component to implement this behavior, but it's not going to be as robust as it would be if supported directly here. Of course, styling an expansion panel to behave correctly when used as a list item is only one approach, and I don't mean to suggest a specific design. My custom component tracks its own expansion state and handles divider logic, etc, internally.

[Char2sGu]

Too many components are missing in this library...

[angular-robot]

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