Page List allows insertion of any child block

Whilst testing the auto inserted Page List in the Nav block we noticed it's possible to insert any block type within the Page List block itself.


https://user-images.githubusercontent.com/444434/205035551-52933ad6-be8c-4e3d-9a3c-9ef6b29f7970.mp4

Page List should only allow `core/page-list-item`.