Mono/C#: Add option to export assemblies outside of PCK

When using this options, assemblies will be saved in the Assemblies folder of the  data directory: 'data_AppName/Assemblies/'.

modules/mono/godotsharp_dirs.h

@@ -56,6 +56,8 @@ String get_project_csproj_path();
 
 String get_data_editor_tools_dir();
 String get_data_editor_prebuilt_api_dir();
+#else
+String get_data_game_assemblies_dir();
 #endif
 
 String get_data_mono_etc_dir();