Does Google Apps Script support external IDEs?

Google has dropped support for Apps Script inside Eclipse but you can set up a local development environment inside VS Code using the Apps Script Starter kit and CLASP.

Here's a video tutorial to help you get started.

As of today (May 2018), every scripts that you add on Google App Scripts (including scripts on Google Drive like spreadsheets) will get accessible through https://script.google.com and will have a specific id that you can get through the url by opening it.

By using clasp which handle a project (like git), you can do:

clasp clone {id}

to have your script in a local folder. Then, after editing your file with your favorite text editor, upload it back with :

clasp push