[two_dimensional_scrollables] Add pinned extent assertions

[Piinks]

Fixes flutter/flutter#136833
Should probably land #5039 first

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[Piinks]

Will update to show a render overflow error instead of throwing a fatal crash. 👍

[stuartmorgan-g]

@Piinks Is this still something you are working on?

[Piinks]

Nope! Thanks for the reminder it was still here. Working on a different solution