This is actually an answer that I searched wildly for 6 months ago, but couldn’t find an explanation for - there was simply and practically no information on how to import and write a (multi or single file; especially for multiple files) JavaScript / Custom Code project - that you wrote yourself - and make it work within Webflow.
Hint: It’s pretty easy to do now:
I created something called CodeSpaces for Webflow → GitHub - BarthMedia/codespaces-for-webflow: How to use GitHub Codespaces for Webflow Designer and create scalable native TypeScript, JavaScrip, jQuery, CSS & Sass applications for your projects.
It leverages the existing infrastructure of GitHub, GitHub Codespaces, Visual Studio Code and the JavaScript package bundler Parcel, allowing you to easily test, run (& produce) your script ideas (you can even use the latest JavaScript features as they are automatically converted to older JavaScript versions).
I hope I’ve provided enough information to get started (in the GitHub README.md file).
I wish you the best of luck and am happy to answer any questions you may have.
P.S…
If you are looking for a more advanced version, this might also be interesting for you → GitHub - BarthMedia/TypeSpaces-for-Webflow: Template repository for building rich native JavaScript / jQuery applications for Webflow