Pages panel: reflect page hierarchy

[jameskoster]

Follow-up to #50463.

Pages are currently displayed at the same level, regardless of structure.

wp-admin	Pages panel

It would be nice to reflect the hierarchy in the Pages panel, ideally with List View's collapsing mechanism:

[jameskoster]

Added this to the 6.3 board as a nice-to-have, but probably fine to punt.

[ndiego]

@jameskoster at this point, do you think we should punt? There seem to be many more pressing issues on the list 😅

[jameskoster]

Agreed, at this stage we'll need to punt.

[bph]

How does everyone feel about getting a fix into 6.4?