Providing a shell.nix file for more conveniently fetching the compatible Zig version

[sstitle]

When trying out the examples for this repo, I was unable to run them on my machine due to a version mismatch with the most up-to-date version of Zig (v0.15.1).

In my opinion, it would be convenient to have a nice way to pull the correct version of Zig compatible with the repo for new users to have a more seamless experience when trying out the repo.

This is a "shell.nix" file for the 0.4 tagged branch. It looks for the .zigversion file in the repo root and fetches the appropriate version of Zig and makes it available on the command line. You can enter the Nix shell by running nix-shell from the repo root as long as you have Nix installed. I like to install Nix using the convenient installer that you can find here from Determinate Systems.

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