[FIX] hooks: useSubEnv supports arbitrary descriptors in env

Before this commit, when defining a getter in the env passed to useSubEnv,
the value was read, losing the definition of the property.

After this commit, declaring a getter in the env works as expected:
the property stays a getter.

src/hooks.ts

@@ -38,7 +38,9 @@ export function useEnv<E extends Env>(): E {
  */
 export function useSubEnv(envExtension: Env) {
   const node = getCurrent()!;
-  node.childEnv = Object.freeze(Object.assign({}, node.childEnv, envExtension));
+  const env = Object.create(node.childEnv);
+  const descrs = Object.getOwnPropertyDescriptors(envExtension);
+  node.childEnv = Object.freeze(Object.defineProperties(env, descrs));
 }
 
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