QueryStartAtConstraint
A QueryStartAtConstraint is used to exclude documents from the start of a
result set returned by a Firestore query.
QueryStartAtConstraints are created by invoking {@link (startAt:1)} or
{@link (startAfter:1)} and can then be passed to {@link (query:1)} to create a
new query instance that also contains this QueryStartAtConstraint.
class QueryStartAtConstraint extends QueryConstraint {}
readonly type: "startAt" | "startAfter";