Implement scroll up operation

[craigthomas]

This PR implements the scroll up operation for the XO chip specification. Unit tests added to match new functionality. This PR closes #44

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 94.93%. Comparing base (b802ca0) to head (a347737).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #46      +/-   ##
==========================================
+ Coverage   92.87%   94.93%   +2.05%     
==========================================
  Files           4        4              
  Lines         660      691      +31     
==========================================
+ Hits          613      656      +43     
+ Misses         47       35      -12     

Files	Coverage Δ
chip8/cpu.py	98.97% <100.00%> (+2.52%)	⬆️
chip8/screen.py	100.00% <100.00%> (ø)

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