Allow wrapping and variable interpolation in `help` in the devshell menu

[deemp]

Based on #290

Interpolation

There's a .env file not added to git.

ADDRESS="http://localhost:3000"

A devshell user would like to open the address in a browser.

She can write this expression.

devShells.default = devshell.mkShell {
  bash.extra = ''
    set -a
    source .env
  '';
  devshell.menu.interpolate = true;
  commands = [
    { 
       package = "hello"; help = "open $ADDRESS";
    } 
  ];
};

And get this devshell menu.

🔨 Welcome to devshell

[[general commands]]

  hello - open http://localhost:3000
  menu  - prints this menu

[devshell]$

Wrapping

The default devshell looks like this:

🔨 Welcome to devshell

[[general commands]]

  menu                    - prints this menu

[packages]

  diffutils-3.10          - Commands for showing the differences between
                            files (diff, cmp, etc.)
  goreleaser-1.23.0       - Deliver Go binaries as fast and easily as
                            possible

[scripts]

  nix fmt                 - format Nix files
  nix run .#bench         - Run benchmark
  nix run .#docs          - Run mdBook server at "http://localhost:3000"

[utilites]

  golangci-lint-1.55.2    - golang linter
  hub-unstable-2022-12-01 - GitHub utility

[devshell]$

[deemp]

Hmm, what can be the error on macOS?

UPD: it was due to longer than devshell.menu.width lines on macOS

[deemp]

@zimbatm, @blaggacao, please, review

[blaggacao]

Although this is not a moderated repository. Recent Moderation Team actions has scared me away from the Nix Community.

I'm sorry, won't be available for review. This has nothing to do with you and your effort which is really commendable. Keep the spirit up!