Extend ScrollOffsetController to current position and scroll without animation

[slawekkrol]

Description

This PR extends ScrollOffsetController to additional two functions, get current position of scroll and scroll without animation. This is motivated by my own project which I needed move scroll instead of animate (you can't use Duration.zero on scroll animation) conditionally when position >= some item position. To summarize this PR supports both.

Related Issues

#41
#151
#293
#415

Checklist

Before you create this PR confirm that it meets all requirements listed below by checking the relevant checkboxes ([x]). This will ensure a smooth and quick review process.

• I signed the CLA.
• All tests from running flutter test pass.
• flutter analyze does not report any problems on my PR.
• I am willing to follow-up on review comments in a timely manner.

[MagTuxGit]

Could you also add this:
ScrollPosition get position => _scrollableListState!.primary.scrollController.position;
Need to get
scrollOffsetController.position.maxScrollExtent