If you want to try it, you can find my patches at
gtksourceview: https://gitlab.gnome.org/wwarner/gtksourceview
Builder: https://gitlab.gnome.org/wwarner/gnome-builder/-/tree/wip/wwarner/semantic-highlighting
My Builder downstream almost certainly has many C/GLib things wrong, because I don't know what I'm doing. If anyone wants to take a look at it and help me out, that would be nice.