Add Nimby - Unofficial package manager and installer.

[treeform]

See: https://github.com/treeform/nimby

When I added Nim to our company CI, our builds suddenly became very slow. Nimble installs took almost two minutes for Fidget2. That felt wrong, so I started digging.
I tried replacing Nimble with a few simple shell scripts that just cloned the repos with git. It built fine, and was way way faster! 2 minutes vs 3 seconds faster.
So Nimby started from a simple idea: download Nim packages from git, do not resolve dependencies, and in parallel.