This plugin provides a code minimap next to the usual scrollbar in the script editor.
This is a regular editor plugin. Copy the contents of addons/zylann.code_minimap into the same folder in your project, and activate it in your project settings.
This plugin is working with several hacks, because the API this would normally require does not exist yet, so the code is not as good as I would like to write it, and some basic functions are missing (such as colors). I made it initially as a proof of concept, but if people like it the needed functions could be requested in Godot.