- Add `load_resource_with` extension function which inserts a loadable
resource by value without requiring implementing `FromWorld` trait.
- Add singleton-like `ManifestResource` which holds a `ManifestAsset`
of a given asset type. Once loaded, the resource is added, just like
`load_resource` did previously.
- Add a `Manifest` system param with a helper lookup function to make
looking up entries easier as you'd otherwise need 3 system params.
- Add a `InitManifest` extension trait for easy setup
- Add a `blocks.manifest.ron` file that references all blocks
- Implement `ManifestEntry` on `BlockDefinition` and `BlockVisuals`
- Remove `BlockDefinitionCollection` and `BlockVisualsCollection`
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