Create zeitschrift-fuer-deutsche-philologie.csl

This should fulfill this <a href="https://forums.zotero.org/discussion/21423/style-request-zeitschrift-fuer-deutsche-philologie-zfph-german-humanities/">old style request</a>.

Please note
 * I suggest in this style to use encyclopediaArticle for "Werke in Editionen" and the style will then print the "Ders./Dies." instead of the editors (which are then the same as the authors). I will ignore gender-specific wording here.
 * Normally, only footnotes are used and not bibliography. However, this csl style implements also a bibliogrpahy and should at least fulfill the needs for "Literaturangaben in sprachwissenschaftlichen Beiträgen" (point 10).

@adam3smith Thank you for the good starting point "Kritische Ausgabe (German)"!
I'm surprised this even validates (@rmzelle ?) but just use
```
<date variable="issued">
<date-part name="year"/>
</date>
```