separate out IsSubType from IsValidImplementationFieldType

[yaacovCR]

Motivation: IsSubType could be re-used within ResolveAbstractType when its behavior is clarified. See: #973.

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[leebyron]

Nice!

Confirming that this aligns to the existing https://github.com/graphql/graphql-js/blob/main/src/type/schema.ts#L311 in the ref impl.

This IsValidImplementationFieldType() is in large part directly mapping to https://github.com/graphql/graphql-js/blob/a24a9f35b876bdd0d5050eca34d3020bd0db9a29/src/utilities/typeComparators.ts#L38 which maps well to the change you proposed