[DevTools] Create extension panels before React detection (#37151)

The main reason why I am doing this is because there is no API in the
browser to "unmount" the created panel.

If you have React DevTools installed, we should always create a panel,
but the contents of the panel should be dynamically populated based on
the target. If it is not a React app, we will continue showing the stub
message.

Previously, we wouldn't mount a panel at all, and historically we've
received a few reports of this as a bug.