the `devmode` helper made for the Nixpkgs/NixOS manual was exposed wrapped
in `mkShell`, which made it impossible to reuse.
this change strips that wrapper and reproduces it at the call site.
now one can use `devmode` from anywhere Nixpkgs is available:
devmode = pkgs.callPackage "${pkgs.path}/pkgs/tools/nix/web-devmode.nix" {
buildArgs = toString ./.;
open = "/index.html";
};
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