feat: add a flake module for use via flake.parts

[brianmcgee]

Based on the suggestion from @terlar

Closes #238

Allows users to use devshell within a flake.parts based flake like so:

{inputs, ...}: {
    imports = [
        inputs.devshell.flakeModule
    ]; 

    perSystem = {
        devshell.shells.default = {
            env = [ ];
            commands = [ ];
            packages = [ ];
        };

        devshell.shells.projectA = {
            env = [ ];
            commands = [ ];
            packages = [ ];
        };

        devshell.shells.projectB = {
            env = [ ];
            commands = [ ];
            packages = [ ];
        };
    };
}

[brianmcgee]

bors merge

[bors[bot]]

Build succeeded:

• build (macos-latest)
• build (ubuntu-20.04)
• devShell default [x86_64-linux]
• docs (ubuntu-20.04)
• Evaluate flake.nix
• flakes (ubuntu-20.04)

[roberth]

Rendered docs: https://flake.parts/options/devshell.html