allow line returns when enter to submit mode is enabled

[reobin]

when vim_enter_submit is enabled, it would be great to have shift+enter break line, or have the option to with a config.

[leohenon]

This should already work via keybinds.input_newline.

{
  "vim_enter_submit": true,
  "keybinds": {
    "input_newline": "alt+return"
  }
}

I will add this to the docs for clarity.