Double-tap to zoom to paragraph width

A common feature of web browsers is that double-tapping on a made-for-desktop website zooms so that the tapped element fills the screen. We can enable a similar behaviour in BookReader using the text layer to set the zoom to the width of the tapped paragraph, making for a consistent experience that makes reading on mobile easier.

Start by locating the BookReader text-layer handling and the existing zoom and tap interaction entry points. Implement the described double-tap behavior for a paragraph and verify that the tapped paragraph fills the screen consistently on mobile.

Written by the indexing model from the issue text.