Runtime config / schema validation

Some users (typically users who don't yet use TypeScript) are running into problems due to misconfigurations of their Contentlayer config (especially in regards to the content schema). Additionally to the safety provided by TypeScript, we should also implement a runtime validation step to avoid undefined runtime errors.

## Technical notes

Ideally we should figure out a way how to derive [Zod](https://github.com/colinhacks/zod)-like validation logic based on the existing TypeScript definitions. We could also investigate to use [TS+](https://github.com/ts-plus) for this.

## Related issues

- https://github.com/contentlayerdev/contentlayer/issues/193