[FIX] app: support for arbitrary descriptors in env

Before this commit, when defining a getter in the env passed to the App,
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/app/app.ts

@@ -44,7 +44,8 @@ export class App<T extends typeof Component = any> extends TemplateSet {
       console.info(DEV_MSG);
     }
     if (config.env) {
-      this.env = Object.freeze(Object.assign({}, config.env));
+      const descrs = Object.getOwnPropertyDescriptors(config.env);
+      this.env = Object.freeze(Object.defineProperties({}, descrs));
     }
     if (config.translateFn) {
       this.translateFn = config.translateFn;