Refactor triangle to use calc()

[roginfarrer]

triangle() currently expects height and width values that are a string value with a unit (e.g. '12px') or a number (12). It throws an error otherwise.

This prevents the use of CSS custom variables. However, if we change from using JS arithmetic to CSS calc(), we can support CSS variables, in addition to number of unit values, without having to do any unit stripping in strings.

[codecov[bot]]

Codecov Report

Merging #616 (a40a1de) into main (275e646) will not change coverage. The diff coverage is 100.00%.

@@            Coverage Diff            @@
##              main      #616   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
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  Files           89        89           
  Lines          860       858    -2     
  Branches       323       320    -3     
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- Hits           860       858    -2     

Flag	Coverage Δ
unittests	100.00% <100.00%> (ø)

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Impacted Files	Coverage Δ
src/mixins/triangle.js	100.00% <100.00%> (ø)

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