ui: allow custom esbuild plugins

packages/ui-default/types.ts

@@ -1,7 +1,6 @@
 /* eslint-disable @typescript-eslint/no-unused-vars */
 declare let UserContext: any;
 declare let UiContext: any;
-declare let Hydro: any;
 // eslint-disable-next-line camelcase
 declare let node_modules: any;
 declare let LANGS: Record<string, import('@hydrooj/utils/lib/lang').LangConfig>;