[Sync EN] Enable runnable (WASM) examples for Predefined Interfaces & Classes (language/predefined)

PR: `https://github.com/php/doc-en/pull/5485` (2026-05-05)

**Archivos ES a actualizar**
- `language/predefined/arrayaccess.xml`
- `language/predefined/fiber.xml`
- `language/predefined/fibererror.xml`
- `language/predefined/interfaces.xml`
- `language/predefined/iteratoraggregate.xml`
- `language/predefined/serializable.xml`
- `language/predefined/unitenum/cases.xml`

