alt Hacker NewsNot sure about Unity, bot Godot is already used to build tools, like Pixelorama (pixel art graphics editor, a bit akin to Asesprite), RPG In A Box (game engine targeted for RPG games), Bitmapflow (tool to generate in-between animation frames), and probably more I don't know about.
Well, if I remember correctly, the Godot editor is written in Godot.
Godot is written in C++ It may have some GDScript in there, but I don't think so. The sourcecode is available: https://github.com/godotengine/godot