Add callback for tap on highlight (decoration) #82

Hello.

I am using the Flutter Readium plugin in my project and it works great for most reading features.

However, I noticed that there is no reliable callback for tap on a highlight decoration that would return the decoration ID.

For my use case this is very important because I need to open a highlight menu or edit a highlight when the user taps on it.

I tried implementing this behavior in my own fork by adding gesture handling on top of the WebView, but this approach is not reliable because the navigator internally handles taps and gestures.

What I would like to ask is whether it would be possible to add a native callback when a highlight is tapped, which returns the highlight (decoration) ID.

For example something like:

func onHighlightTap(decorationId: String)