Add Book Title to BookExport/Printer Friendly Version

Problem/Motivation

It's not a problem but it would be nice to be able to include {{ book_title }} in addition to the exported page title to the title tag in the document head as well as to use it in text above the exported node content in book-node-export-html.html.twig

Steps to reproduce

• Create a book.
• Create child pages.
• Browse to child page.
• Click Printer-friendly Version.
• Page title only reflects the child page but does not indicate what the name of the book.