Add TypeScript types for helpers

**Feature request**
It would be great to have serialization for core.

**Use case**
Many users need this functionality for production deployments.

**Implementation idea**
Add a new config option `enable_serialization` with default `false`.

No files, tests, or entry points are named. First reconcile the TypeScript-types request with the body’s serialization and enable_serialization proposal, then locate the relevant helper and configuration entry points. Before implementation, define the intended scope and acceptance tests so completion can be verified.

Written by the indexing model from the issue text.