In pandoc 2.7 we assumed that every class with chapters would
accept `\frontmatter`, `\mainmatter`, and `\backmatter`.
This is not so (e.g. report does not). So pandoc 2.7
breaks on report class by including an unsupported command.
So we replace the book-class variable in the template with
two variables, has-chapters and has-frontmatter, and set
these intelligently in the writer.
Closes#5348.
John MacFarlane
ebdcf75276