Refactor renderers and RenderResult (#46955)
This PR does two major changes:
1. Make sure both pages renderer and app renderer return `RenderResult`,
no more `null`. This was achieved with a new `null` type in the
constructor `new RenderResult(null)`, and a `.isNull()` method.
2. Remove all mutations of the `renderOpts` object inside renderers. To
pass extra information out, they need to be attached to the
`RenderResult` now. This also requires 1) to be done.
These changes are the initial steps to the isolated rendering worker
architecture. Besides those there're also some type improvements.
Fixes NEXT-807.
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