Add .yml to Citeproc formatFromExtension (#7706)

Make Citeproc recognize files with .yml extension (in addition to .yaml)
as YAML bibliographies.

Closes #7707.

src/Text/Pandoc/Citeproc.hs

@@ -357,6 +357,7 @@ formatFromExtension fp = case dropWhile (== '.') $ takeExtension fp of
                            "bib"      -> Just Format_biblatex
                            "json"     -> Just Format_json
                            "yaml"     -> Just Format_yaml
+                           "yml"      -> Just Format_yaml
                            _          -> Nothing