Use modules_enabled.gen.h to improve inter dependency checks

- Fix build with gdscript module disabled. Fixes #31011.
- Remove unused `gdscript` compile option.
- Fix build with regex module disabled.
- Fix ImageLoaderSVG to forward declare thirdparty structs.

main/tests/test_string.cpp

@@ -28,15 +28,19 @@
 /* SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.                */
 /*************************************************************************/
 
-#include "core/ustring.h"
-#include <wchar.h>
-//#include "core/math/math_funcs.h"
+#include "test_string.h"
+
 #include "core/io/ip_address.h"
 #include "core/os/os.h"
-#include "modules/regex/regex.h"
-#include <stdio.h>
+#include "core/ustring.h"
 
-#include "test_string.h"
+#include "modules/modules_enabled.gen.h"
+#ifdef MODULE_REGEX_ENABLED
+#include "modules/regex/regex.h"
+#endif
+
+#include <stdio.h>
+#include <wchar.h>
 
 namespace TestString {