delegate body validation

[ignatiusmb]

A continuation to #1997 (comment), TypeScript implicitly infers index signature for type aliases, hence why we got an error when using it with interfaces.

That's why to counteract those differences, we'd need to wrap the interfaces with Typify helper for the index signature workaround, but I guess we could also have us wrap it directly in our types without any visible downsides, yet.

Closes #1997

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[ignatiusmb]

The only reason this is drafted is because I was planning to include some documentations for the types section, if the docs can wait in another and/or this looks good enough as-is, feel free to mark it as ready.

[Rich-Harris]

Wow, this is clever! Personally I don't know that we need to document this — the name BodyValidator is self-explanatory, and anyone curious about the definition can see it at /docs/types#additional-types-bodyvalidator

[dummdidumm]

Smart!