DevTools: Support mulitple DevTools instances per page (#22949)

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).