This can be used to make particles disappear when colliding, rather
than bouncing around or sticking to surfaces.
This is useful for rain particles which should not be able to go
through floors/ceilings, but shouldn't stick to surfaces either.
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.
GPUParticles attractors and collision are currently only available in 3D.
Their 2D counterparts haven't been implemented yet, but they will use
separate nodes.
