To avoid speculations regarding the speed of creation of all lexers, I created https://github.com/geany/geany/pull/3673 testing it (less than 2 ms on Raspberry Pi 4). So I don't think we have to worry about speed here.
Thank you, but my intended point was that it should be done offline before release (if not in CI), not when being used by a user, thats too late.
I didn't read the discussion above, but I totally support replacing builtins with configurability.