Add new style for German journal "Deutsche sprache"

The guidlines are quite short and let some issues unclear. Therefore, I checked the style also by looking at a current article closely. (It is very probable that this was asked in https://forums.zotero.org/discussion/39057/citation-style-request-deutsche-sprache/ )
> Okay, et-al-use-first="1", but that is the same as what it actually is doing at the moment (standard? visual editor thing?). 

I think it may default to that by accident, but I think a close reading of the specs could be read to mean that you don't get et-al unless that's specified. So in light of other implementations I'd leave it as is.

> Moreover, I should do this in all author, editor, creator macros (yes, I see examples for that now). Was this the meaning of your 2nd sentence?

Yes. If that's universal, it's better to set this on cs:citation and cs:bibliography rather than the individual name macros.

> Moreover, I should do this in all author, editor, creator macros (yes, I see examples for that now). Was this the meaning of your 2nd sentence?