Updating mono. Adding MSBuild.

[Brett Gilio]

Hi all,

Our mono package is pretty severely out of date. I want to take care of
updating it to a version consistent with this century. However, there
seems to be an issue perhaps along the lines similar to bootstrapping
Rust and JDK.

Mono, for most distributions, seems to be bootstrapped with a prebuilt
binary mono-lite. Due to this, I am unsure of how to make the first step
in correctly repackaging Mono.

Additionally, this should ideally come with the newer MSBuild, which Nix
currently lacks. It is included in the Mono source tree, but I would
assume we would want that to be separated and rather called by a
propagated/native input?

Anyways, a lot to unpack here. I'd appreciate any help I could get.

Best,
Brett Gilio