Remove react/react-dom/etc from externals

1e26dfd2b160d4b4218efed0ac0c41f7db8f0829 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?)

Review commit 1e26dfd2b160d4b4218efed0ac0c41f7db8f0829 and the current externals configuration to understand how react, react-dom, and moment are handled. Done means users can control these scripts again without an unintended hard dependency on WordPress core; determine from the existing configuration whether a setting is needed.

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