Audit for disabling text selection

inspired by https://news.ycombinator.com/item?id=21804316

Like `font-size` does, we could flag pages that have >X% of text w/ text selection disabled. This feature is antagonistic to the user. We should leave some room for unselectable text, as there are some use cases for it, but in general that should be a fraction of the text on the page.

We may way to generalize `FontSize` to a `Text` artifact.