To create a new editor tool, use test.lua from /lua/ge/extensions/editor/ as a starting template, name your file accordingly and deleted not needed code from it. The main function is the onEditorGui which is called by the editor when it needs to draw and process GUI events (in our case imgui). Here you will be adding your GUI calls to create the tool, no limitations on how you layout your GUI, you can create your own windows and so on. If your tool exposes events to the extension mechanism, then you can create hooks named after the template: onEditor*, like for example: onEditorVehicleCreated, onEditorCloudDelete.

Editor tools will show their exposed hooks in the header of the Lua file as comments.