registerPhysicsBackend function Physics
Registers factory under backend id, replacing any existing entry.
A document's physicsWorld component names its backend by id; backend
packages register their factory at app startup (registerPhysicsBackend( 'rapier3d', RapierWorld.new)) so documents authored against them realize.
The pure-Dart basic backend is preregistered.
Implementation
void registerPhysicsBackend(String id, PhysicsBackendFactory factory) {
_physicsBackends[id] = factory;
}