[add] adjust webpack config feature

[hakobpogh]

For testing, I've installed react-scripts and react-dev-utils from my local copy of the code.

This change is adding a small feature which will let users to not eject from react-scripts for just a tiny webpack modification. In my case, I needed to change the config.target only and I didn't want to get rid of react-scripts for that.

I hope we can discuss the way you want this and we can have it in the next release :)

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[hakobpogh]

@iansu @mrmckeb @ianschmitz @petetnt please review my PR. The failed pipelines are not from me because they're related to the parts I haven't changed. They're all throwing the following error: Module not found: You attempted to import /home/vsts/work/1/create-react-app/packages/react-dev-utils/refreshOverlayInterop.js which falls outside of the project src/ directory. Relative imports outside of src/ are not supported.

[notaphplover]

Maybe those test fails are related to #9738

[hakobpogh]

When I was recently ejecting React, I found this:

NOTE: Create React App 2+ supports TypeScript, Sass, CSS Modules and more without ejecting: https://reactjs.org/blog/2018/10/01/create-react-app-v2.html

The feature I'm suggesting is just going to help with this... People will not eject for the small issues, but instead, they will use this feature.