Add standalone plugins to the wp-env config

[joemcgill]

Summary

This updates the included .wp-env.json config file to ensure all standalone plugins are installed on the environment during development and testing. This behavior can be overridden locally using a .wp-env.override.json file.

See #911

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Checklist

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Co-authored-by: joemcgill <joemcgill@git.wordpress.org>
Co-authored-by: felixarntz <flixos90@git.wordpress.org>
Co-authored-by: thelovekesh <thelovekesh@git.wordpress.org>
Co-authored-by: westonruter <westonruter@git.wordpress.org>
Co-authored-by: mukeshpanchal27 <mukesh27@git.wordpress.org>

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