This adds a `isFallback` property to detect if the page is being rendered in "fallback" mode or normal mode.
Accessed via the `useRouter()` hook.
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Closes#10527
* Add initial SSG fallback handling
* Remove extra changes and update fallback handling
* Remove extra timeout for testing
* Update SSG tests in dynamic-routing suite
* Add racing to decide between rendering fallback and data
* Update size-limit test
* Update comment
* Make sure to follow correct route change order
* Make comment more verbose for racing
* Revert getStaticData to only return Promise
* Make sure to update URL on fallback
* Add retrying for data, de-dupe initial fallback request, and merge fallback replace
* Update to add preload for fallback pages data
* Add test for data preload link
* Use pre-built fallback in production mode
* Remove preload link for fallback from _document
* Update to make sure fallback is rendered correctly for serverless
