[FR] Make all subsection of an unnumbered chapter unnumbered.

[cderv]

Aim is to avoid having 0.1, 0.2 in headers when you add a first unnumbered section. Like a Preface with sub headers.

Manually, this could be done by adding unnumbered too all sub headers. With a Lua filter, we could do it automatically.

This could also be something to put in rmarkdown directly. And even in Pandoc itself in a way.

cc @apreshill as we mentioned this.