feat(flake-module)!: a flake check for each devshell

[mightyiam]

Here is a nix flake show

git+file:///home/mightyiam/s/numtide/devshell?dir=templates/flake-parts
├───checks
│   ├───aarch64-darwin
│   │   └───"devshells/default" omitted (use '--all-systems' to show)
│   ├───aarch64-linux
│   │   └───"devshells/default" omitted (use '--all-systems' to show)
│   ├───i686-linux
│   │   └───"devshells/default" omitted (use '--all-systems' to show)
│   ├───x86_64-darwin
│   │   └───"devshells/default" omitted (use '--all-systems' to show)
│   └───x86_64-linux
│       └───"devshells/default": derivation 'devshell'
└───devShells
    ├───aarch64-darwin
    │   └───default omitted (use '--all-systems' to show)
    ├───aarch64-linux
    │   └───default omitted (use '--all-systems' to show)
    ├───i686-linux
    │   └───default omitted (use '--all-systems' to show)
    ├───x86_64-darwin
    │   └───default omitted (use '--all-systems' to show)
    └───x86_64-linux
        └───default: development environment 'devshell'

And here are some questions:

• Should this be on by default?
  I think it should, but that would add flake outputs to existing users when they bump this input.
  Not sure that's acceptable in this community.
• Would adding this require also adding an option to enable this?
• This should also be documented when other questions are answered.
• Tests? Not sure I see any existing flake-module tests...