A “global default script” already exists as the onFirstSceneLoaded lifetime function in extensions. You usually can paste the bundled code of a library into a JS event in that lifetime function to have it loaded in.
Well we leave to you the liberty to chose your toolchain. If you want to bundle or use any tooling on the exported js files, you are welcome to do so. I don’t think forcing tooling onto people is a good idea. I hope in the future to add plugins to my gdexport project (a cli exporter for GDevelop games) to help streamline the process of building your own toolchain.