Adds validate function to attributes

[rpaul-stripe]

This PR addresses issue #379. It adds support for writing a custom validate function for an individual schema attribute. The function can return an arbitrary number of Markdoc validation error objects.

After weighing the tradeoffs, I decided not to support async functions for this case because it would require us to collect and await all of them before continuing, introducing more complexity. We can still potentially add support for that in the future if there's sufficient demand.

[mfix-stripe]

@rpaul-stripe I thought we chatted about this previously. I feel like this unnecessarily expands our API surface area, when custom types already support this case, and lead to better reuse.

@matv-stripe @nkohari-stripe what do you think about this proposal?

[rpaul-stripe]

@mfix-stripe I think there's value in being able to do one-off validation functions independently of the type attribute without having to create a reusable type. And I think it's consistent with the way that the validation function works on the schema object, so it doesn't feel like it clutters up the API. We can provide guidance to encourage people to still use custom types in cases where reuse is desirable.

I have been thinking a lot about how we want to handle complex object/array attribute values. Rather than adding a bunch of complexity to Markdoc's type system to explicitly support more feature-rich validation on nested values, what I want to do is provide a pattern that makes it really easy for users to pull in Zod or JSON Schema for those cases. I think a validate function on the attribute is ultimately a good way to handle this.

[mfix-stripe]

Ack, won't block your vision then 👍