Upgrade to Bevy v0.12 (#12)

* Migrate `System::pipe(...)` calls to `System::map(...)`
* Migrate `EventReader::iter()` and `EventReader::into_iter()` calls to
  `EventReader::read()`
* Migrate asset handling
* Generate `*.meta` files for assets
* Migrate a shader import
* Fix emissive materials double-applying their emissive strength
* Fix non-normalized bone weights resulting in incorrect vertex positions
* Set shadow filtering to `Hardware2x2` to mitigate increase in shader
  compilation times

.vscode/settings.json

@@ -6,4 +6,7 @@
 	"rust-analyzer.cargo.features": [
 		"wasm"
 	],
+	"files.associations": {
+		"*.meta": "ron"
+	}
 }