The icons here are optimized SVGs, because the editor renders the svgs at runtime, they need to be small in size, so they can be efficiently parsed.
The original icons can be found at: https://github.com/godotengine/godot-design/tree/master/engine/icons
There you can find the optimizer script.
If you add a new icon, please make a pull request to this repo: https://github.com/godotengine/godot-design/
and store the optimized SVG version here.