feat: Freeze cache after parsing and return it

This allows accessing the parsed values straight from the
page component. Freezing also ensures the object reference
stays immutable for the whole duration of the page render.

packages/next-usequerystate/src/cache.ts

@@ -20,8 +20,9 @@ export function createSearchParamCache<
     const c = getCache()
     for (const key in parsers) {
       const parser = parsers[key]!
-      c[key as Keys] = parser.parseServerSide(searchParams[key])
+      c[key] = parser.parseServerSide(searchParams[key])
     }
+    return Object.freeze(c)
   }
   function getSearchParam<Key extends keyof Parsers>(
     key: Key