vim-patch:9.2.1035: filetype: Github citation files are not recognized (#41643)

Problem:  filetype: Github citation files are not recognized
Solution: Detect *.cff files as yaml filetype (Wu Zhenyu).

Reference:
https://docs.github.com/en/repositories/managing-your-repositorys-settings-and-features/customizing-your-repository/about-citation-files

closes: vim/vim#21163

https://github.com/vim/vim/commit/05de89440166f045535ebf06fb7aeec962f3fbd2

Co-authored-by: Wu, Zhenyu <wuzhenyu@ustc.edu>

runtime/lua/vim/filetype.lua

@@ -1492,6 +1492,7 @@ local extension = {
   yy = 'yacc',
   ['y++'] = 'yacc',
   yxx = 'yacc',
+  cff = 'yaml',
   yml = 'yaml',
   yaml = 'yaml',
   eyaml = 'yaml',