Split Orbital backend out into winit-orbital (#4243)

.github/CODEOWNERS

@@ -27,5 +27,4 @@
 /src/platform_impl/windows          @notgull
 
 # Orbital (Redox OS)
-/src/platform/orbital.rs            @jackpot51
-/src/platform_impl/orbital          @jackpot51
+/winit-orbital                      @jackpot51

.github/workflows/ci.yml

@@ -182,6 +182,10 @@ jobs:
     - name: Test winit core
       run: cargo test -p winit-core
 
+    - name: Test winit Orbital
+      if: contains(matrix.platform.target, 'redox')
+      run: cargo test -p winit-orbital
+
     # Test only on Linux x86_64, so we avoid spending unnecessary CI hours.
     - name: Test dpi crate
       if: >