feat(Emitter): add ParticleEmitter

[mcgivrer]

Add a new component to the Particle System to handle the particle emitter side.

• create ParticleEmitter interface to generate particles for ParticleGoup.
• add an emitters list attribute to the ParticleSystem
• add initialization phase to ParticleSystem, ParticleGroup to split concern from create ParticleGroup and generate Particle's.
• add emit() method to the ParticleGroup to activate its internal emitter.