scrollable_positioned_list: expose position, offset and add jumpToOffset in ScrollOffsetController

[yakagami]

Description

Exposes ScrollPosition and offset of the primary Scrollable within ScrollOffsetController. Also adds jumpToOffset which is the same as animateScroll but without animation (you cannot set Duration to zero so this is necessary). Exposing position allows us to use jumpToOffset as just jumpTo by subtracting the initial offset. Exposing ScrollPosition is useful for it's drag method which is similar to animateScroll but can be used with DragUpdateDetails from eg. onVerticalDragStart in a GestureDetector.

Fixes #513

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[yakagami]

Looks like someone made a similar PR already here: #487. I think exposing position is also important as noted in the comments there.

[yakagami]

Looking into this further it seems jumpToOffset doesn't work for large offsets as explained here: #460 (comment). It worked for my use case as well as in the test that I added here though