The extension doesn’t work with pseudo-elements

This is caused by a bug on Chromium’s end which I can’t work around: https://crbug.com/334984154

The bug being that when selecting an element inside an iframe, the reported value for `$0` returns `undefined` for any pseudo-element.

Because this bug needs to be fixed on Chrome’s end, I am marking this issue as https://github.com/bramus/containing-block-devtools-extension/labels/external and https://github.com/bramus/containing-block-devtools-extension/labels/wontfix (more like _“cantfix”_, right?). To make sure this issue can be found, I’m keeping this one open and am also adding it to the https://github.com/bramus/containing-block-devtools-extension/labels/FAQ.