navbar-vertical-next: alternative collapse mode

[spike-rabbit]

The navbar-vertical-next should implement an alternative collapsed mode (can be changed via input).

<si-navbar-vertical-next inlineCollapse>
  ...
</si-navbar-vertical-next>

See #1943 for visual details.

Since rendering the active item might be complicated, I suggest a two step approach:

1. PR just implement the new collapse button + behavior but don't show any other items in collapsed mode
2. PR implement showing showing the active item
3. PR implement the expanding into a list in the collapse mode