feat: Add is_at_start and is_at_end methods

[DeflateAwning]

Fixes #1018.

This is still a work in progress. Looking for help/feedback with the tests.

For this to work reliably and as expected, I believe that the Scroll object needs to "write back" to this ScrollbarState object, so that this ScrollbarState understands its viewport size. This communication is at the heart of most of the scrollbar deficiencies, as far as I can tell.

[codecov]

Codecov Report

Attention: Patch coverage is 66.66667% with 2 lines in your changes are missing coverage. Please review.

Project coverage is 89.4%. Comparing base (b7778e5) to head (7242b2b).

Files	Patch %	Lines
src/widgets/scrollbar.rs	66.6%	2 Missing ⚠️

Additional details and impacted files

@@          Coverage Diff          @@
##            main   #1024   +/-   ##
=====================================
  Coverage   89.4%   89.4%           
=====================================
  Files         61      61           
  Lines      15430   15434    +4     
=====================================
+ Hits       13799   13803    +4     
  Misses      1631    1631           

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