Don't unset keys like --font-weight-* when unsetting --font-*

[adamwathan]

This PR fixes an issue where unsetting --font-* would unset --font-weight-* and --font-size-*.

@theme {
  --font-weight-bold: bold;
  --font-size-sm: 14px;
  --font-sans: sans-serif;
  --font-serif: serif;
}
@theme {
  --font-*: initial;
  --font-body: Inter;
}

Up until now this was sort of intended/desired behavior but with recent changes there are now more overlapping theme keys (--inset-* and --inset-shadow-* as well for example), and we don't want to make it impossible to unset just the default font-family values.

This PR also simplifies how we were handling making sure that the inset-* utility ignored --inset-shadow-* variables since it's all really the same problem.

This does mean we need to maintain a list of known theme keys so we know when there is a conflict between two keys, which is kind of unfortunate because up until now this was a totally dynamic thing. End users can still add whatever custom stuff they want under @theme but we don't really know about those namespaces since we're maintaining a static list so we can't resolve conflicts there. I'm confident there are ways we could solve this if it actually becomes a problem, so content to push forward without solving it right now and just deal with it if/when it actually arises, because it just might not.