[YAML] Add CITATION.cff file extension

[jwortmann]

https://citation-file-format.github.io/

Also see https://github.com/citation-file-format/citation-file-format/blob/main/schema-guide.md which states that the file must be named exactly CITATION.cff and that it is YAML.

[jfcherng]

It's spec says CCF is in yaml format and the only reasonable usage seems to be CITATION.cff files.
https://citation-file-format.github.io/assets/pdf/cff-specifications-1.0.0.pdf

[deathaxe]

Would it make sense to add it to hidden_file_extensions as it points to a whole file, which is unlikely to be used as normal file extension and thus should be hidden from "Save as" dialog?

[jwortmann]

Yes, I think that would be better. Currently it shows up as *.CITATION.cff if the "Save as" dialog, which could be misleading. I'll change it, unless there are any objections.