Export QueryStream (#712)

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I'm creating Effect wrappers around Convex and ConvexHelpers, and while
working on the stream helpers, I struggled a lot with TypeScript because
I don't have access to the type of the internal `QueryStream` class. If
it's ok to export it that would help me avoid having to do castings to
`any` 😅.
(If not, that's fine)

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Author: ianmacartney

packages/convex-helpers/server/stream.ts

@@ -214,7 +214,7 @@ type GenericStreamItem = NonNullable<unknown>;
 /**
  * A "QueryStream" is an async iterable of query results, ordered by indexed fields.
  */
-abstract class QueryStream<T extends GenericStreamItem>
+export abstract class QueryStream<T extends GenericStreamItem>
   implements GenericOrderedQuery<T>
 {
   // Methods that subclasses must implement so OrderedQuery can be implemented.