fix: Restrict `:has`, `:not`, and others to selector arguments

[savetheclocktower]

(fixes #45)

This isn't the elegant solution I was hoping for, but it's the smallest change that I could make work. Since :has, :not, :is, and :where are the only pseudo-classes that take selector arguments, we can prevent the conflict by disallowing plain values inside of those four altogether.

I bet there's a simpler solution, too, but I'm submitting this just in case.