This PR adds the system option to the theme selector:

• Main menu item ToggleTheme accepts new props:

  props:
    | {
        allowSystemTheme: true;
        theme: "light" | "dark" | "system";
        onSelect: (theme: "light" | "dark" | "system") => void;
      }
    | {
        allowSystemTheme?: false;
        onSelect?: (theme: "light" | "dark") => void;
      },

This should ensure that it works as it did until now, but it also allows the onSelect handler to be specified, which will reflect the change to the host app. Also, it adds an allowSystemTheme prop, which will change the button to a button set (similar to one used for selecting scale in the export dialog. In this case, passing the current host app theme prop is also necessary as the editor itself is unaware of the system theme.

• The Excalidraw app now has a new reducer and useEffect to handle theme changes and a listener for changing the system theme.

• The Excalidraw app is now grabbing keyboard shortcut for theme change and sends the change down to the editor.

• Script in the HTML of the Excalidraw app now also reflects the system theme when selected and stored in localStorage