Add pause/resume

[angelorodem]

With this feature, integrators can pause the emission and wait the particles finish smoothly, and later resume if needed.

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[angelorodem]

Related to issue #26

[angelorodem]

Tested this code with

    var effect = Newton(
      key: newtonKey,
    );

    var rainEffect = RainEffect(
      particleConfiguration: ParticleConfiguration(
        shape: CircleShape(),
        size: const Size(15, 15),
        color: SingleParticleColor(color: COULUMB_BLUE),
      ),
      effectConfiguration: EffectConfiguration(
          particleCount: 0,
          minDuration: 3000,
          maxDuration: 7000,
          particlesPerEmit: 3,
          maxFadeInLimit: 5,
          minFadeInLimit: 4),
    );

    if (appState.getIsCharging()) {
      print("Started animation");
      if (newtonKey.currentState!.isEffectsListEmpty()) {
        newtonKey.currentState?.addEffect(rainEffect);
      } else {
        print("Effects not empty");
      }

      newtonKey.currentState?.resumeEffects();
    } else {
      print("Stopped animation");
      newtonKey.currentState?.pauseEffects();
    }