Implement getServerSnapshot in userspace shim (#22359)

* Convert useSES shim tests to use React DOM

Idea is that eventually we'll run these tests against an actual build of
React DOM 17 to test backwards compatibility.

* Implement getServerSnapshot in userspace shim

If the DOM is not present, we assume that we are running in a server
environment and return the result of `getServerSnapshot`.

This heuristic doesn't work in React Native, so we'll need to provide
a separate native build (using the `.native` extension). I've left this
for a follow-up.

We can't call `getServerSnapshot` on the client, because in versions of
React before 18, there's no built-in mechanism to detect whether we're
hydrating. To avoid a server mismatch warning, users must account for
this themselves and return the correct value inside `getSnapshot`.

Note that none of this is relevant to the built-in API that is being
added in 18. This only affects the userspace shim that is provided
for backwards compatibility with versions 16 and 17.

scripts/error-codes/codes.json

@@ -395,5 +395,6 @@
   "404": "Invalid hook call. Hooks can only be called inside of the body of a function component.",
   "405": "hydrateRoot(...): Target container is not a DOM element.",
   "406": "act(...) is not supported in production builds of React.",
-  "407": "Missing getServerSnapshot, which is required for server-rendered content. Will revert to client rendering."
+  "407": "Missing getServerSnapshot, which is required for server-rendered content. Will revert to client rendering.",
+  "408": "Missing getServerSnapshot, which is required for server-rendered content."
 }