Nested folders: Paginate child folder items

[ashharrison90]

What is this feature?

• extends pagination behaviour to child folder items
  • adjusts useLoadNextChildrenPage hook to return a function which requires the folder uid as the argument
    • this then allows for it to be called for different folders, not just the root folder for the page
    • handleLoadMore is passed the startIndex requesting the load. we can then find this item in the list, find the parentUID of this item and load more items from the correct parentUID 🥳
• adjusts when pagination placeholders are added
  • before, these were only added at the root
  • now they are also added for child folders that are open whose collection isn't fully loaded
• consolidates PAGE_SIZE and ROOT_PAGE_SIZE into 1 variable: PAGE_SIZE

Why do we need this feature?

• better performance when opening a child folder

Who is this feature for?

• anyone using nested folders

Which issue(s) does this PR fix?:

Fixes #70574

Special notes for your reviewer:

was expecting this to be much trickier... logic is very simple but seems to be working pretty well. still testing some edge case scenarios.

Please check that:

• It works as expected from a user's perspective.
• If this is a pre-GA feature, it is behind a feature toggle.
• The docs are updated, and if this is a notable improvement, it's added to our What's New doc.

[joshhunt]

Neat stuff. I've been playing around with it trying to break it but it seems to work pretty good!

Scrolling past an unloaded folder and then back up is a bit funky, but i'm not really sure there's much we can do about that - we would really need to know how many items are in a folder to get the proper amount of placeholders to prevent the weird scroll jumps.

I'm glad to see how straight forward this was to add on to what was there before 🎈