Fix infinite recursion in theme variable lookups

[thecrypticace]

Fixes #1463

We'll have to bail in some cases that we didn't before to fix this. This seems fine though. The core problem was that given this theme:

@theme {
  --radius: calc(var(--radius));
}

We'd see this CSS:

.rounded {
  border-radius: calc(var(--radius));
}

And we'd try to replace variables in calc(var(--radius)). This doesn't start with a var(…) so our recursion guards didn't catch this.

This entire system is a bit fragile and I'll be working to clean this up along with how we handle pixel equivalents, light-dark replacements, color-mix replacements, etc… in some future PRs (#1330 is one of those but I want to reorganize + tweak some code first in an earlier PR which will make that one "nicer")