So, the "microworld" builder fork is here:
https://github.com/bromagosa/Snap/tree/microworlds
Check out the documentation here:
https://github.com/bromagosa/Snap/blob/microworlds/MICROWORLDS.md
And a live instance can be found here:
But if what you want is JUST a custom dropdown menu for a block input, that can be done in Snap! itself. Let me know if that's the case.