Allow to use bold label by default for theorem env in non HTML or PDF output.

[cderv]

This would be a way to resolve #1202 and not reintroducing the :

This makes them same style as PDF or HTML output

This probably needs more testing but I think this will correctly only apply on non HTML / non PDF output. However, once bold is the default, there is currently no way to change using a config.

We could make passing a function more generic (https://github.com/rstudio/bookdown/issues/1202#issuecomment-883224612) to apply also on environments. This way the default function could be overwritten.

Creating this as draft for discussion