A sandbox game that allows you to create and save structures with boxes, circles, and more.
Swirl is available on the web here. Alternatively, there is an offline Windows desktop app available here.
If you'd like, you can write a custom script for Swirl to do whatever you want. There's some documentary for the swirl API available to scripts, but you're also welcome to look at the source. You can find more information on the wiki.
While index.html should run out of the box, you can also build your own copy of the Windows desktop app. Just download the source, run npm install, and run npm start to boot it up or npm run-script make to build an installer.