zloirock/core-js ? reverse-engineered prompt

Build me a JavaScript standard library polyfill package that I can drop into older browsers or Node projects.

I want it to cover modern language features like Promise, Symbol, collections, iterators, typed arrays, Array helpers like flat and flatMap, and newer web APIs like URL and structuredClone when possible. It should be modular so I can import only the pieces I need, and it should also have a version that does not add anything to the global scope. Please make sure it works cleanly with Babel style usage too, since I want to use it in real apps without a lot of setup.

If some features depend on current specs or edge cases, look up the latest docs online if you need to. Also include the basic docs and examples so someone can understand how to install it and use it quickly.