It was previously possible to open a JSON file that was created by downloading an Apps Script file, back into the Apps Script editor, but I don't see a way to do that now. Now, when FILE - OPEN is chosen, the G Suite Developer Hub opens, and the file list doesn't list JSON files.
I could probably rig up a way to run it through the Apps Script API and create a new Apps Script file, but I guess there is no way to do it manually any more.