Code quality: Fix header guards consistency

Adds `header_guards.sh` bash script, used in CI to validate future
changes. Can be run locally to fix invalid header guards.

scene/3d/mesh_instance_3d.h

@@ -28,8 +28,8 @@
 /* SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.                */
 /*************************************************************************/
 
-#ifndef MESH_INSTANCE_H
-#define MESH_INSTANCE_H
+#ifndef MESH_INSTANCE_3D_H
+#define MESH_INSTANCE_3D_H
 
 #include "core/templates/local_vector.h"
 #include "scene/3d/visual_instance_3d.h"
@@ -98,4 +98,4 @@ public:
 	~MeshInstance3D();
 };
 
-#endif
+#endif // MESH_INSTANCE_3D_H