This makes it easier to spot syntax errors when editing the
class reference. The schema is referenced locally so validation
can still work offline.
Each class XML's schema conformance is also checked on GitHub Actions.
It's easy to assume they are the same, but they are quite different for the two types of 2D lights. For myself, it took a bit of confusion and experimentation for me to figure out why this behaviour changed when I changed from point to directional. Hopefully it can save somebody else the trouble.
