Allow listening to primary ScrollController

[Lootwig]

Adds an optional callback to ScrollablePositionedList that provides access to updates on the primary ScrollController.

This enables reading the ScrollPosition of the controller, in my case required for checking whether a list still fully fits on the screen or is scrollable already.

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[Lootwig]

Note, I'm starting to think whether it might make even more sense to just allow injecting your own ScrollController instead - I noticed that if the parent widget of the List changes size, the controller doesn't fire a notifyListeners if the list isn't actually scrolled, which means I have to handle this case differently anyway via callbacks triggered when the parent rebuilds (and reading the controller's value then, because they do update)