Masonry: update layout logic to support rendering skeleton pins

[rebeccayeyi]

Summary

What changed?

• Added a utility function to retrieve an item's height from the skeleton pin, using it instead of the measurement cache when applicable.

Why?

• This change is necessary to render skeleton pins as part of Masonry's items rather than _loadingStateItems.
  • This should help with overall performance since we are now having the real pins just replace the skeleton pins where currently, we render either the skeleton pins or the real pins
  • This also unlocks infinite scrolling

Test Plan

• Tested https://github.com/pinternal/pinboard/pull/105265 with local Gestalt

Note

• If this change is okay, will also apply the same changes to Masonry v2!

Checklist

• Added unit tests
• Added documentation + accessibility tests
• Verified accessibility: keyboard & screen reader interaction
• Checked dark mode, responsiveness, and right-to-left support
• Checked stakeholder feedback (e.g. Gestalt designers, relevant feature teams)

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[rebeccayeyi]

thoughts on making sure that the pin is a skeleton pin? we could just check to see if height is available, but i wasnt sure if we wanted to expose that API