Add missing type

packages/bun-types/bun.d.ts

@@ -2390,6 +2390,11 @@ declare module "bun" {
      */
     unref(): void;
 
+    /**
+     * Flush any buffered data to the socket
+     */
+    flush(): void;
+
     /**
      * Reset the socket's callbacks. This is useful with `bun --hot` to facilitate hot reloading.
      *