Set up a place for body validations & add the first new check

[dylan-apollo]

A bunch of refactoring to set us up for body validations—and adds a single check, if you use empty $args with no path but have no arguments defined on the field.

This doesn't use the visitor stuff because we're interested in the opposite info—where's the data coming from, not where is it going to. Left a bunch of TODOs where we should add more validations.

[svc-apollo-docs]

✅ Docs Preview Ready

No new or changed pages found.

[dylan-apollo]

We're working on a more general interface for iterating over mapping inputs/outputs. We'll use that for validating bodies instead of the AST directly