Serialization through Bevy's type registry

[djeedai]

This change transitions from the typetag dependency to using Bevy's own
built-in TypeRegistry to serialize and deserialize Hanabi's EffectAsset (*.effect).

This solves a long-standing issue where modifiers couldn't be serialized easily,
and user-provided modifiers were ignored for serialization. With the use of
Bevy's own type registry, all built-in and user modifiers are treated equally,
as long as they are registered via register_reflect_modifier(). All built-in modifiers
are automatically registered by the HanabiPlugin.

That registration allows leveraging the new ReflectModifier type data, which
provides a factory function to create a concrete modifier type instance. This is
used during deserialization to instantiate the actual modifier object. This works
around the lack of reflection/serialization support for Box<dyn Trait>.

This change has a few knock-on effects on other parts of Hanabi:

• EffectAsset::mesh is now an Option<AssetPath<'static>> referencing the
  mesh, if any, used to render particles. That path is now also serialized.
• At runtime, the new EffectMesh component can override this asset mesh.

With this, EffectAsset is now entirely serializable. This doesn't solve the entire
asset story, as some additional items are still missing (notably, textures). But
this should greatly improve the value proposition for loading Hanabi effects
from assets on disk, rather than generating effects in code.