[core] Add "Document" query field, returning any document with given ID

[rexxars]

Type of change (check at least one)

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation (fix or update to documentation)
• Maintenance
• Other, please describe:

Does this change require a documentation update? (Check one)

• Yes
• No

Current behavior

Currently you have to know which type a document is before querying for it with a DocumentTypeName query.

Description

This PR introduces a Document field (matches the interface name, is a reserved type for Sanity Studio, and matches the other DocumentTypeName queries). It has the same, single argument of a document ID, and has the return type of Document, which means you'll have to use conditional fragments to query on non-document properties.

In time we should also see if we can make some kind of psuedo-filter for all types, which would allow allDocument(...), but that's a much larger task.

Note for release

• Introduce new Document field on GraphQL APIs for fetching any document by ID.

Checklist

• I have read the Contributing Guidelines
• The PR title includes a link to the relevant issue
• The PR title is appropriately formatted: [some-package] PR title (#123)
• The code is linted
• The test suite is passing
• Corresponding changes to the documentation have been made

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[saasen]

LGTM! 👍