Add Gio app shells and platform stubs (T-003)

app-modern/platform/windows/platform_stub.go

@@ -0,0 +1,19 @@
+//go:build !windows
+
+package windows
+
+import "runtime"
+
+type platformStub struct{}
+
+func newPlatform() Platform {
+	return platformStub{}
+}
+
+func (platformStub) OS() string {
+	return runtime.GOOS
+}
+
+func (platformStub) Edition() Edition {
+	return EditionModern
+}