Extract Win32 runtime session helper

src/platform_windows/windows_runtime_session.h

@@ -0,0 +1,14 @@
+#pragma once
+
+#include "platform_windows/windows_bootstrap_helpers.h"
+#include "platform_windows/windows_splash.h"
+
+namespace pp::platform::windows {
+
+void run_main_window_runtime_session(
+    const MainWindowStartupState& startup,
+    bool start_in_vr,
+    HINSTANCE instance,
+    SplashScreen& splash);
+
+}