Refactor generic code in reorderable_list.dart into a base class / widget

[navaronbracke]

Use case

I was looking into implementing a sliver reorderable grid, similar to the existing SliverReorderableList.
During my research, I found that a lot of code in the existing widget is private, but generic enough to be applicable to a SliverReorderableGrid (which is what I am trying to implement).

The fact that this code is private makes creating my own widget a little harder.
I have to essentially copy-paste all the private code and reimplement the important parts.

Proposal

Refactor the generic code in reorderable_list.dart into a more generic set of base classes, i.e. SliverReorderable, _DragItem<T extends ReorderableState>, in a file like reorderable.dart. The private code could still be kept private, but moved to that file.
The existing SliverReorderableList should be refactored to use this new SliverReorderable.

This would make fixing #46430 easier, as the only thing missing is the SliverReorderableGrid, which would extend the SliverReorderable.

cc @darrenaustin