workspace: add experimental pnpm support

packages/hydrooj/src/plugin-api.ts

@@ -1,4 +1,9 @@
+import serialize from 'serialize-javascript';
 import db from './service/db';
+
+export { nanoid } from 'nanoid';
+export { serialize };
+
 export * from './pipelineUtils';
 export * from './error';
 export * from './libs';