Merge the functionality of Custom Book Block to Book 3

Hi, per 🌱 Plan for Book 3.0 release Active , this is a feature request to merge the functionality of Custom Book Block → module into the Book 3 release.

The module provides these additional features, which all seem quite non intrusive and could easily just be a part of the main module:

• Choose to show only a single book (or default to show all, as in core)
• Choose to dynamically detect the current book
• Choose a starting level and a maximum depth to create the menu
• Force the menu to be expanded

Also note MR in 🐛 Title and top level settings are not respected Active where the UI settings in Custom Book Block is more aligned with the UI settings in Book module.