DoublyLinkedList: Implement `find` in terms of `AK::find`

Problem:
- The implementation of `find` is coupled to the implementation of
  `DoublyLinkedList`.
- `append` and `prepend` are implemented multiple times so that
  r-value references can be moved from into the new node. This is
  probably not called very often because a pr-value or x-value needs
  to be used here.

Solution:
- Decouple the implementation of `find` from the class by using a
  generic `find` algorithm.
- Make `append` and `prepend` be function templates so that they can
  have binding references which can be forwarded.

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