oh-my-zsh: add 'preLoaded' option (#351810)

            Without this option it would default to the read-only nix store.

          '';

        };

        preLoaded = lib.mkOption {

          type = lib.types.lines;

          default = "";

          description = ''

            Shell commands executed before the `oh-my-zsh` is loaded.

            For example, to disable async git prompt write `zstyle ':omz:alpha:lib:git' async-prompt force` (more information https://github.com/ohmyzsh/ohmyzsh?tab=readme-ov-file#async-git-prompt)

          '';

        };

      };

    };

          ZSH_CACHE_DIR=${cfg.cacheDir}

        ''}

        ${cfg.preLoaded}

        source $ZSH/oh-my-zsh.sh

      '';