Add prerender indicator for app router (#67306)
## Background
Currently in app router during development all pages are dynamic
rendered and streamed since we don't know if a page will be prerendered
until after the page is rendered. During build time we do a render pass
for each path and if it doesn't bail from static generation it is
considered prerendered/ISR'd.
Since it can be tricky to track all the bail outs for static generation
and doing numerous builds to track this isn't feasible we want to have
some sort of signal in development to show if it would have been
prerendered.
## New Handling
To alleviate the above pain point we are planning to bring back the
prerendered indicator from pages initially and have a config to disable
if desired. We are also going to investigate other possible indicators
potentially through headers or similar.
<details>
<summary>Preview</summary>
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Co-authored-by: Jiachi Liu <inbox@huozhi.im>
