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.
The physics API cannot provide needed functionality to ensure the correct behavior, which might lead to confusion (see `rest_info()`).
However `get_closest_collision_safe/unsafe_fraction()` methods are not removed, because they return correct result from `cast_motion()`.
