Guard against infinitely recursive theme key lookup

[thecrypticace]

When constructing the design system we compute color swatches for all classes. This process involves:

• Computing the CSS for a utility
• Inlining any values from the theme, recursively
• Scanning for CSS color syntax that we can compute a swatch for

We guard against directly self-recursive replacements because of how the replacement itself functions. However, if you wrapped the variable in something then we would effectively recurse into the expression and try to replace the variable again. This caused the expansion to then be wrapped and then try again resulting in an infinite loop.

This fixes it by keeping track of seen variables during a replacement and if we've seen it and the expansion contains a var(…) anywhere we'll only replace it once. This logic is not perfect as the thing that matters is that the replacement itself results in a recursion but this is good enough for now.

Fixes #1329