Allow for arbitrary and multiple selectors in aria- variant definitions

[MichaelAllenWarner]

Right now, the aria- variant settings only allow for simple single-string values that get inserted into the `&[aria-${normalize(value)}]` string here: https://github.com/tailwindlabs/tailwindcss/blob/master/src/corePlugins.js#L414-L432

This is a bit limiting.

For example, it's quite sensible to define a custom aria-current variant with a selector like &[aria-current]:not([aria-current="false"]), but currently this couldn't be achieved in the aria theme-setting (would have to be done in separate custom addVariant()/matchVariant() plugins).

Likewise, out of the box, Tailwind's aria-pressed and aria-checked variants both get "true" values for their corresponding attribute, but both attributes also support a "mixed" value (in addition to "true" and "false"). One could conceivably want a single variant to cover the non-"false" cases, in which case again you'd need separate custom addVariant()/matchVariant() plugins with a selector like &[aria-pressed]:not([aria-pressed="false"]).

I propose adding an advanced option for the aria- variant definitions, where the user can supply a value whose type is the same type as the second parameter for addVariant()—namely, a selector or array of selectors with the & syntax. This would give users greater flexibility without having to define their more complex aria- variants elsewhere in the config file.

If the Tailwind folks are interested in the idea, I could probably find time to contribute here.

[MichaelAllenWarner]

I suppose that because of the peer- and group- definitions, the "advanced" selectors I'm proposing here should be limited to those that modify a single element (no combinators, at-rules, etc.). And no & necessary in that case (maybe even prohibit it).