+" Escape the shell argument based on the shell.
+" Vim and Neovim's shellescape() are insufficient.
+" 1. shellslash determines whether to use single/double quotes.
+" Double-quote escaping is fragile for cmd.exe.
+" 2. It does not work for powershell.
+" 3. It does not work for *sh shells if the command is executed
+" via cmd.exe (ie. cmd.exe /c sh -c command command_args)
+" 4. It does not support batchfile syntax.
+"
+" Accepts an optional dictionary with the following keys:
+" - shell: same as Vim/Neovim 'shell' option.
+" If unset, fallback to 'cmd.exe' on Windows or 'sh'.
+" - script: If truthy and shell is cmd.exe, escape for batchfile syntax.