Group variant stacking

[RobinMalfait]

This PR will make it possible to stack group variants together. It will make sure that all the states like focus and hover are applied to the .group correctly.

HTML:

<div class="group">
  <div class="group-focus:group-hover:font-bold"></div>
</div>

Will generate:

.group:focus:hover .group-focus\:group-hover\:font-bold {
  font-weight: 700;
}

---

This is a jit only feature.