Fix infinite loop

[zsoerenm]

Fixes #871
slotParentNode is always the same parentNode:

const slotParentNode = el?.assignedSlot?.parentNode;

Therefore, target for next round doesn't change. This ends in an infinite loop.
I'm unsure what the purpose of

const parentNode = slotParentNode || targetParentNode;

was.
Setting

const parentNode = targetParentNode;

works in my case, but I'm unsure, if it breaks others code.

[daybrush]

@zsoerenm

lit-movaeble's new version is released. Thank you for PR :)