Add support for .cff (Citation File Format) extension as YAML

[MattF-NSIDC]

Resolves #434

[asottile]

searching this extension leads to ambiguity so unfortunately it cannot be accepted

[MattF-NSIDC]

Understandable!

For future searchers looking to do this: for my project it was not straightforward / I didn't know how / have time to figure out how to get pre-commit to run prettier on my CITATION.cff file without also running it on a bunch of other stuff I don't want it to, like Markdown, or having to maintain a regex to exclude all the other possibilities identified as "text".

I ended up using the cffconvert pre-commit hook which is designed specifically to validate this file. I decided to live without prettier 🤷