Keep server code out of browser chunks (#76660)
Reduces the main chunk's size for a hello world app ~from 60.2 kB to
51.6 kB~ from 53.5 kB to 45.6 kB (after #76622 got merged).
In the past we were a bit sloppy with using modules from `src/server`
also in universally used modules (server & browser). This led to code
ending up in browser chunks that's only meant for the server, e.g. async
local storage modules.
We did have already quite a few conditional requires with `typeof
window` checks for those modules, but not everywhere. This is fixed in
this PR. And shared modules are also moved out of `src/server` into
either `src/shared/lib` or `src/client`.
A simple smoke test is added to ensure we don't regress here, at least
for a simple hello world app. A better verification might be some kind
of linter rule though.
In addition, we don't remove the `typeof window` SWC optimization for
Next.js modules anymore (which is still done for other `node_modules`,
see #62051), to ensure that guarded code becomes dead code and is
eliminated.
Ideally, SWC would take care of the DCE solely by using `typeof window`
checks, without needing to manually split modules and dynamically
requiring them, which is very tedious and we'd like to avoid. But this
is currently not the case.
Another option worth exploring is using the ESM modules from
`next/dist/esm` in hopes of yielding better DCE results.
