Support border-opacity with default border color

[adamwathan]

Resolves #4261.

This PR moves the rules for adding the default border color out of preflight directly and into the borderColor plugin using addBase, making it possible to use our withAlphaVariable helper to ensure that borderOpacity utilities work with the default border class without adding a specific border-color utility. I think this actually makes way more sense anyways.

There are ways in which this is a breaking change but I am really not convinced it's a problem. It would only be a breaking change for people using Preflight without using the rest of Tailwind, or for people using Tailwind with the borderColor core plugin disabled for some reason but still expecting to be able to use border to get a border with the default border color. If this is even one person I would be amazed.

[codecov-commenter]

Codecov Report

Merging #4277 (a71cf25) into master (218e57f) will decrease coverage by 0.01%.
The diff coverage is 70.00%.

@@            Coverage Diff             @@
##           master    #4277      +/-   ##
==========================================
- Coverage   71.70%   71.68%   -0.02%     
==========================================
  Files         220      220              
  Lines        4725     4733       +8     
  Branches      780      782       +2     
==========================================
+ Hits         3388     3393       +5     
- Misses       1257     1259       +2     
- Partials       80       81       +1     

Impacted Files	Coverage Δ
src/plugins/borderColor.js	60.00% <70.00%> (+1.17%)	⬆️

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