Background
In the babylon.js serialization system, many kind of objects have doNotSerialize, to keep object from being serialized.
But currently IParticleSystem and its implements do not have a doNotSerialize, or any other props to do this.
Proposal
Add an optional doNotSerialize to IParticleSystem, and impl it in ParticleSystem and GPUParticleSystem with a default false, so existing code would not be broken, and since it’s optional, compatiblity is preserved for typescript users making their own impls.
In SceneSerializer._Serialize, doNotSerialize would be checked like all other type of objects for IParticleSystems.