provide declarations for server-only/client-only (#80361)
We implement the behavior of `import 'server-only'` and `import
'client-only'` on the compiler level and thus don't require having them
installed as dependencies. By default it works fine with typescript,
because (surprisingly) TSC *doesn't check side-effecting imports*.
But this behavior can be overridden with
[`noUncheckedSideEffectImports`](https://www.typescriptlang.org/tsconfig/#noUncheckedSideEffectImports)
which'd cause `import 'server-only'` to start erroring. To prevent that,
we add declarations for them in next's global type declarations.
