fix(nuxt): Added top-level fallback exports (#18083)

It seems that in some cases where an `environment` in Vite is set to
something other than `node` or `browser`, it won't be able to figure out
the correct import paths.

This PR ensures that when Vite (or other tools) can't determine the
environment from "browser" or "node" conditions alone, then it falls
back to the server bundle similar to what we do in Next.js.

In the issue's case it was set to `nuxt` or rather
`vitest-environment-nuxt` which doesn't seem like a pattern to account
for.

closes #18070

Author: logaretm

packages/nuxt/package.json

@@ -27,7 +27,9 @@
       "node": {
         "import": "./build/esm/index.server.js",
         "require": "./build/cjs/index.server.js"
-      }
+      },
+      "import": "./build/esm/index.server.js",
+      "require": "./build/cjs/index.server.js"
     },
     "./module": {
       "types": "./build/module/types.d.ts",