feat: Add support for laying out graphs horizontally

[mlyoung101]

For my project, the graphs work much better when laid out horizontally, so I added this as a feature upstream. It just adds a flag to io::to_dot called horizontal, that defaults to false to preserve existing behaviour. Tests added too :)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.58%. Comparing base (8b0701d) to head (b82914d).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #301   +/-   ##
=======================================
  Coverage   99.58%   99.58%           
=======================================
  Files          59       59           
  Lines        2621     2637   +16     
  Branches      136      137    +1     
=======================================
+ Hits         2610     2626   +16     
  Misses         11       11           

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