[DevTools] e2e Regression Testing App

[lunaruan]

This PR adds an e2e regression app to the react-devtools-shell package. This app:

• Has an app.js and an appLegacy.js entrypoint because apps prior to React 18 need to use ReactDOM.render. These files will create and render multiple test apps (though they currently only render the List)
• Moved the ListApp out of the e2e folder and into an e2e-apps folder so that both e2e and e2e-regression can use the same test apps
• Creates a ListAppLegacy app because prior to React 16.8 hooks didn't exist.
• Added a devtools file for the e2e-regression
• Modifies the webpack config so that the e2e-regression React app can use different a different React version than DevTools

Open Question @bvaughn: Can this replace the e2e folder, or is it worth it to keep the e2e folder around for now? I'm inclined to leave and delete later.

[bvaughn]

Open Question @bvaughn: Can this replace the e2e folder, or is it worth it to keep the e2e folder around for now? I'm inclined to leave and delete later.

I don't have much of an opinion on this. Whatever you think is best.

I just skimmed this because I don't want to hold up your progress, but I don't have much extra brain power right now to context switch to this. If you want a more thorough review, maybe Mengdi can do it or I could do it in a day or two?