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
This is being done so that we can embed DevTools within the new React (beta) docs.
The primary changes here are to `react-devtools-inline/backend`:
* Add a new `createBridge` API
* Add an option to the `activate` method to support passing in the custom bridge object.
The `react-devtools-inline` README has been updated to include these new methods.
To verify these changes, this commit also updates the test shell to add a new entry-point for multiple DevTools.
This commit also replaces two direct calls to `window.postMessage()` with `bridge.send()` (and adds the related Flow types).
