feat: endAt impl

Author: MichaelVerdon

packages/firestore/lib/modular/query.js

@@ -140,6 +140,14 @@ export function endAt(...fieldValues) {
   return new QueryConstraint('endAt', ...fieldValues);
 }
 
+/**
+ * @param {DocumentSnapshot} snapshot
+ * @returns {QueryEndAtConstraint}
+ */
+export function endAt(snapshot){
+  return new QueryConstraint('endAt', snapshot);
+}
+
 /**
  * Creates a QueryEndAtConstraint that modifies the result set to end before the provided fields relative to the order of the query.
  * The order of the field values must match the order of the order by clauses of the query.