feat: add nix flake.nix

[RobWalt]

Connections

• the existing shell.nix file

Description

There are currently two major ways of using nix: With and without flakes. Currently the repo only hosts a shell.nix file which supports the non-flake way of running nix stuff like development shells. This PR adds a flake.nix and a modularized directory nix where people can find the implementation of the flake-devshell.

• Compared to the shell.nix file, this development shell only includes the necessary dependency to get the wgpu examples running. People are free to extend this minimal devshell and tailor it to their own needs.
• The flake approach also supports locking dependencies which helps with the reproducibility of everything.

Testing

• I entered the devShell on my machine and ran the hallmark example successfully. ✅