Add flake.nix for devshell

[indium114]

As it says on the tin

[NXTler]

I'm not familiar with NixOS and don't really see the purpose for this. Could you please elaborate?

[indium114]

I'm not familiar with NixOS and don't really see the purpose for this. Could you please elaborate?

A devshell is essentially a temporary environment which has all of the tools needed for development, which is needed because NixOS discourages polluting your normal environment with development tools. This PR adds a flake.nix file, which outputs a devshell with all of the development tools needed to work on this project.

The .envrc file works with Direnv, so that you don't have to type nix develop to enter the devshell, it will automatically enter it when you cd into the project directory.

The .gitignore entry ignores the .direnv directory, which is a cache for the devshell when used with Direnv