Support validation

[lishaduck]

Describe the feature

I think it'd be helpful for c12 to have validation support built-in via Standard Schema.

Use case: push the parsing as far to the edge of applications as possible. Could possibly provide better errors (i.e, before the config gets defu'd, so that the specific file erroring can get noted)?

Alternative: Continue implementing in userland.

Additional information

• Would you be willing to help implement this feature?