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

studio/assets/skyboxes/README.md.meta

@@ -0,0 +1,4 @@
+(
+    meta_format_version: "1.0",
+    asset: Ignore,
+)