By the way, have you seen
https://github.com/geany/geany/blob/master/HACKING#L528
It might help you when adding a new lexer. Also, I'd suggest that you check some existing PR adding a new lexer such as https://github.com/geany/geany/pull/3934 so you know what needs to be changed (just ignore the tagmanager and ctags part).
Finally, if you plan to develop some "non-standard" lexer like your universal lexer, I'd strongly suggest to consult this with Neil first so that your idea is compatible with what Neil wants to have in Lexilla so you don't waste much time in case he doesn't like it for some reason.