[babel-plugin] refine priorities for pseudos and descendant and sibling selectors

Right now we use a predetermined priority mapping for pseudoclasses and descendant and sibling selectors. Some are intuitive and nondisruptive (:hover loses to :active), others will be confusing. This is especially true for descendant and sibling selector cases.

We probably can't apply a negation like we do for media queries for CSS size reasons (there are too many permutations), though it's something we can investigate.

A simpler approach would be to include an autosort so the user understands the expected prioritization between selectors.