Include `created_at` and `updated_at` in Typescript export

[blackmann]

The created_at and updated_at fields are autogenerated so they're not part of the schema.

We need to hardcode created_at: DateStr¹ (same for updated_at) when exporting collections. Note that, this should only render on the collection level.

Related file: https://github.com/blackmann/mangobase/blob/5a6b6bd082f8803ea90e26048f190add74e4c96f/base/src/lib/export-schema.ts

Footnotes

1. DateStr is just an alias for string ↩