EffectTimer getters, Delay::trigger_immediately, and assorted cleanup.

[AlephCubed]

• Added get_timer, get_mode, and mutable equivelents to EffectTimer trait.
• Added default implementation for EffectTimer::merge.
• Fixed warning message referencing old StatusEffectPlugin instead of AlchemyPlugin.
• Added Delay::trigger_immediately builder method.
  • This is now used in both poison examples.
• Removed register_timer_merge_functions in favor of merge_effect_timer.

// Before:
register_timer_merge_functions(&mut registry);
// After:
registry
    .register::<Lifetime>(merge_effect_timer::<Lifetime>)
    .register::<Delay>(merge_effect_timer::<Delay>);

• Made merge_effect_stacks public.
• Implemented Add<Self>, AddAssign<Self>, From<u8>, and Into<u8> for EffectStacks.
• Various documentation and examples cleanup.