chore: add flutter quickstart example, assets and update README

examples/flutter/quickstart/windows/flutter/generated_plugin_registrant.cc

@@ -0,0 +1,14 @@
+//
+//  Generated file. Do not edit.
+//
+
+// clang-format off
+
+#include "generated_plugin_registrant.h"
+
+#include <thermion_flutter/thermion_flutter_plugin_c_api.h>
+
+void RegisterPlugins(flutter::PluginRegistry* registry) {
+  ThermionFlutterPluginCApiRegisterWithRegistrar(
+      registry->GetRegistrarForPlugin("ThermionFlutterPluginCApi"));
+}