Remove react/react-dom/etc from externals

[rmccue]

1e26dfd configured react/react-dom/moment as externals. This removes the ability for users to control these scripts, and goes against the original purpose of react-wp-scripts (for SPAs) as it introduces a hard dependency on core's version of these scripts.

(Might be worth a setting of some sort for this?)

[roborourke]

I feel like this is a reasonable default considering this version of react-scripts is geared towards WP and has a significant impact on the end bundle size. Happy to have a flag that sets it otherwise though to cover all the bases / use-cases.

[rmccue]

and has a significant impact on the end bundle size

Users end up loading the same amount of data regardless, so it's not too big of an issue on that front.

[roborourke]

Not if something is loading WP's version of React as well. That's really what this is addressing - the plugin use-case.