Add functional color variables from Primer Primitives to the theme

[VanAnderson]

First item in https://github.com/github/design-systems/issues/1219

This PR deep merges in primer primitives into our theme colors and also separates out colors from compound css statements.

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[VanAnderson]

@colebemis thank you so much for sending me that refactor! Your code was definitely a big improvement in readability, using reduce was a much better option. I implemented that and added a couple of unit tests to do a basic sanity check on filtering out Shadows and colors. I can get more in depth with the assertions if needed.

[colebemis]

Looks great! 👍