[JS/TS] fix flatbuffers default export (#6123)

* [TS/JS] Export flatbuffers namespace as default export

* [TS/JS] Fully qualified name

* Export as default and named

ts/flatbuffers.ts

@@ -31,3 +31,4 @@ export namespace flatbuffers {
     export const ByteBuffer = ByteBufferClass;
 }
 
+export default flatbuffers;