Refactor CMake configuration for ImGuiTestEngine and feature flags

- Updated the CMake configuration to use target_compile_definitions for ImGuiTestEngine, improving clarity and maintainability.
- Introduced IMGUI_TEST_ENGINE_DEFINITIONS to streamline the definition process for targets linking to ImGuiTestEngine.
- Modified feature flags in features.h to conditionally enable the native file dialog based on the YAZE_ENABLE_NFD flag, enhancing flexibility in feature management.

src/app/core/features.h

@@ -39,7 +39,11 @@ class FeatureFlags {
     bool kLogToConsole = false;
 
     // Use NFD (Native File Dialog) instead of bespoke file dialog implementation.
+#if defined(YAZE_ENABLE_NFD) && YAZE_ENABLE_NFD
     bool kUseNativeFileDialog = true;
+#else
+    bool kUseNativeFileDialog = false;
+#endif
 
     // Overworld flags
     struct Overworld {