Allow customization of final character to fix R breadcrumbs

[wleoncio]

In R files, the presence of a hash symbol at the end of the section line prevents it from showing up on the VSC breadcrumbs row:

Removing it makes the section render correctly

I've tried a similar approach on different languages (HTML, Python, C++). In these cases, I couldn't even get the section to appear on the breadcrumb at all (and that may be a topic for a different discussion), but at least for R that's the deal.

I can also understand that hard-coding a different final character might break the extension for other languages, so perhaps the safest option here is to allow the user to custimoze the final character, either in general or, ideally, on a per-language basis.