[Feature] Implement FlixelEmitter and FlixelParticle system

[stringdotjar]

Create a robust particle system engine consisting of FlixelEmitter (the source) and FlixelParticle (the individual sprites).

Requirements:

• Explode vs. Stream modes.
• Customizable ranges for velocity, acceleration, rotation, and alpha.
• Color gradients over the particle's lifetime.
• Integration with FlixelSpriteGroup for batching.