v9.1.1

Reverted support for interface mixins. The accompanying upgrade to a newer version of our webidl2 dependency brought along some other subtle breaking change, which is being investigated in jsdom/jsdom#2443. Until we have that sorted out, this release reverts interface mixin support in order to avoid regressions.

This release was broken upon release and cannot be used, as more code needed to be reverted. Use v9.1.2 (or v9.0.1) instead.