Remove resources from scenes

[Trashtalk217]

Objective

Currently, in scenes, resources are stored like this:

(
  resources: {
    "scene::ResourceA": (
      score: 1,
    ),
  },
  entities: {
    4294967297: (
      components: {
        ...
      },
    ),
    4294967298: (
      components: {
        ...
      },
    ),
  },
)

Since resources-as-components (#20934), it has become possible to attach other components to the resource entities. This is very useful for networking and something we want to support. Currently, however, the scene format does not support additional components in the format. This means that this data will be lost when converting a world to a scene and back to a world. This loss can be a pain for people working with resources.

Solution

Since resources are already stored on entities, this PR removes resources: { ... } and instead everything is done on the entity level:

(
  entities: {
    4294967296: (
      components: {
        "scene::ResourceA": (
          score: 1,
        ),
        "bevy_ecs::resource::IsResource": ((12)),
        "replicon::Replicated": () // yay ^_^
      },
    ),
    4294967297: (
      components: {
        ...
      },
    ),
    4294967298: (
      components: {
        ...
      },
    ),
  },
)

Testing

TODO: Add a test, for what happens when a resource is loaded from a scene twice. What should that even do?

[SkiFire13]

"bevy_ecs::resource::IsResource": ((12)),

Note that component ids are not stable across different instances of a bevy app, so loading this scene is likely not gonna work.