Support multiple screencast windows

[antross]

This specifically would improve the experience when clicking on links which open in a new window/tab (e.g. click on the GitHub icon in the top-right of webhint.io).

Currently the screencast stays on the original tab/window so it can appear that clicking on the link did nothing. However you can see the new tab listed in the targets view of the extension (and you can switch the screencast to it using that view).

However, it would be much more obvious what was happening if a new screencast tab opened in VS Code instead (and the old one remained as well). It would also make it easier to debug multiple pages at once (assuming multiple DevTools instances could also be opened).