Control numbering depth like quarto crossref

Adapted numbering level according to quarto's two level cross-referencing mechanism from quarto 1.4 onwards that adds a crossref key to yaml.

For books, custom blocks are numbered by chapter by default. For single documents or websites, the numbering is consecutive. This can be changed with the crossref yaml key:

crossref:
  chapters: true

turns on numbering by section (level 1 header) for other documents than books, and setting crossref.chapter to false in books switches numbering by chapters off.

Number prefix can be overridden by setting a yaml key numberprefix to the desired string value. This also works as div-block attribute.

no worries about parts and appendices

Added fixes and work arounds for books with parts and/or appendix. Earlier version would crash.
Added optional yaml key chapno to specify the chapter letter as prefix for custom numbered blocks in html books, and attribute secno to level 1 headings for pdf books.

v0.1.1: fixes

fixes:

• example was not working due to a shortcut in an html comment
• a recent fix to make references work in quarto book had broken the filter for ordinary documents

v0.1.0: box, number and ship my theorems and friends

Initial release of a (mostly) working quarto extension that numbers, styles and cross references user defined blocks (html divs / latex environments).

There is an issue with list-of-blocks when rendering books which will be fixed in the next release.