Add a method to set the 'MainDotCsDir' when no main.cs is available

Engine/source/platform/platformFileIO.cpp

@@ -620,3 +620,8 @@ DefineEngineFunction( getUserHomeDirectory, const char *, (), , "getUserHomeDire
 {
    return Platform::getUserHomeDirectory();
 }
+
+DefineEngineFunction(setMainDotCsDir, void, (const char* path), , "setMainDotCsDir()")
+{
+   Platform::setMainDotCsDir(StringTable->insert(path));
+}