Added button variant 'accent'

[robinmolen]

Closes #288

Added an accent button variant, and removed the deprecated outline variant.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.86%. Comparing base (859abd7) to head (ea398b5).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #289      +/-   ##
==========================================
+ Coverage   91.82%   91.86%   +0.03%     
==========================================
  Files         200      200              
  Lines        8430     8417      -13     
  Branches     1240     1240              
==========================================
- Hits         7741     7732       -9     
+ Misses        686      682       -4     
  Partials        3        3              

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