fix(gatsby-plugin-netlify-cms): Mark relevant dependencies as externals

[erezrokah]

Fixes #18245
Fixes #17568

I can split this into two PRs (one per issue) if needed.

Main changes:

1. Mark netlify-cms-app and relevant dependencies as externals to avoid re-build during bundle. netlify-cms-app,netlify-identity-widget, react and react-dom are already set up for that since they expose a global variable on the window object which Webpack can reference instead of importing the module.

2. The reason netlify-identity-widget was still included in the main app bundle here is that it didn't pass Gatsby's default thresholds for code splitting. Forcing it into its own cache group makes sure it's not in the app bundle.
   It will still be included in the html as a link tag with a preload attribute, since it is imported as a runtime dependency (though we do know at build time it won't be needed). I actually wrote some code to remove it from the html using a gatsby-ssr.js but then realized it will prevent using the widget all together in other contexts than the plugin (not sure such a scenario exists).
   Along with code splitting the widget when enableIdentityWidget is true, when the widget is disabled it will be ignored completely from the build via the webpack.IgnorePlugin plugin.

Would love to get some feedback and if there is something I can do to make it easier for other people to test it please let me know (thought about publishing it separately).

Also, I'm not sure how to go about versioning.

Update:
Also fixes #15126

[erquhart]

@LekoArts this is good to go 👍

[sidharthachatterjee]

Thank you so much! 🥇

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