Add Auto Scrolling Container

[MyreMylar]

Step 8 of splitting mega-PR #494 into smaller easier to review PRs

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 96.24%. Comparing base (2081310) to head (b8b51fa).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #545      +/-   ##
==========================================
+ Coverage   96.23%   96.24%   +0.01%     
==========================================
  Files          87       88       +1     
  Lines       12416    12452      +36     
==========================================
+ Hits        11949    11985      +36     
  Misses        467      467              

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[MyreMylar]

I'm probably going to merge this as it is for now I think (once I've improved the tests and tested it out in some examples) - but I want to consider before the next version release whether it worth merging this wholly into the vanilla UIScrolllingContainer rather than having both a UIScrollingContainer and a UIAutoScrollingContainer.

The question is if there are benefits to having the split or not. Do users want manual control over scrolling behaviour of scrollable containers - even if they do, can we accommodate that cleanly in one element rather than two....