Added typings for next/config. (#24815)

types/next/config.d.ts

@@ -0,0 +1 @@
+export default function(): {serverRuntimeConfig: any, publicRuntimeConfig: any};

types/next/tsconfig.json

@@ -26,6 +26,7 @@
         "head.d.ts",
         "link.d.ts",
         "router.d.ts",
+        "config.d.ts",
         "test/next-tests.ts",
         "test/next-error-tests.tsx",
         "test/next-head-tests.tsx",
@@ -34,4 +35,4 @@
         "test/next-dynamic-tests.tsx",
         "test/next-router-tests.tsx"
     ]
-}
+}