Implement the unique-id() function

This PR implements the `unique-id` function introduced in Ruby sass 3.3 by returning a random unique CSS identifier.

This implementation is slightly different to the Ruby implement which

> Returns a unique CSS identifier within the scope of a single CSS file.

I'm not sure the scoping serves any particular benefit but maybe @chriseppstein knows better?

Fixes https://github.com/sass/libsass/issues/636. Not sure how best to spec this to be honest.
ditto on https://github.com/sass/libsass/pull/664#discussion_r20570198