feat: p.o.c showing how to make the toolbar extendable

[jasonbahl]

In theory, a 3rd party plugin developer could enqueue a script and call:

const MyCustomButton = () => {
  return <Button onClick={() => { alert( 'My Custom Functionality' ); }} />
}

addFilter( 'wpgraphqlide_toolbar_buttons', ( toolbarButtons ) => {

  toolbarButtons.myCustomButton => MyCustomButton
  return toolbarButtons;
} );

[josephfusco]

We can use @wordpress/hooks here so that this can eventually be ran outside of WordPress and also signal to @wordpress/scripts to add this dependency to *.asset.php.

[josephfusco]

@jasonbahl I have incorporated this POC into #60 Thanks! 🙌🏻