feat[devtools/extension]: show disclaimer when page doesnt run react and refactor react polling logic

[hoxyq]

Changes:

1. Refactored react polling logic, now each .eval() call is wrapped in Promise, so we can chain them properly.
2. When user has browser DevTools opened and React DevTools panels were mounted, user might navigate to the page, which doesn't have React running. Previously, we would show just blank white page, now we will show disclaimer. Disclaimer appears after 5 failed attempts to find React. We will also show this disclaimer if it takes too long to load the page, but once any React instance is loaded and registered, we will update the panels.
3. Dark theme support for this disclaimer and popups in Firefox & Chromium-based browsers

Important: this is only valid for case when React DevTools panels were already created, like when user started debugging React app and then switched to non-React page. If user starts to debug non-React app (by opening browser DevTools for it), we will not create these panels, just like before.

Q: "Why do we poll to get information about react?"
A: To handle case when react is loaded after the page has been loaded, some sandboxes for example.

Before	After

[gsathya]

the old version has an early return here, why remove that?

[hoxyq]

So, this proxy port is opened from content script in isolated environment, which has access to window object, but not DOM.

This condition is usually an edge case, it means that previous port was not disconnected or we were not notified that it was disconnected. In previous implementation we would disconnect new port, which is questionable, but it could save us from situation with many new connections spam.

I think better approach in this case is disconnecting previous port properly and registering new one, connecting it with devtools page (in browser DevTools) port.