Should be trivial since a Prolog variable must begin with a capital letter or an underscore [1](https://github.com/geany/geany/pull/3171#user-content-fn-1-c5a53e0ad69d4a8a7...).
It's not something we need to implement - it's already implemented by the lexer. It's whether we should map it inside `filetypes.prolog` to something else (and what) than `default` or not.