fix(core): portals should be nestable

[CodyJasonBennett]

Revers 549a6bf where portals' keys were hardcoded to their container's UUID, making them unnestable. I've completed the fix to make portals reactive and nestable.

// These should have the same behavior
<>
  {createPortal(<mesh />, scene)}
  {createPortal(<mesh />, scene)}
</>

<>
  {createPortal(<><mesh /><mesh /></>, scene)}
</>

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[drcmda]

why oh why didn't i comment this, there was a reason, something evil. but now i can't remember. 💀 we do have portals in the examples folder though, if nothing breaks it must have been a fluke.