dash: add libedit support
dash: add libedit support

This allows running set -o emacs to get line editing in dash, which is
a major usability improvement.

Closure size before: 31.6M
              after: 35.3M

NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs