Give `json_agg` a type parameter to be used when input type is known

[aleclarson]

This will require the t.json and t.jsonb types to have a type parameter that can narrow the Json type that is used by default in TypeScript result sets.

<T extends t.anyelement>(arg: ColumnRef<T>): Aggregate<T extends t.json ? t.json : t.json<T>>