add support for openscad .scad files

Hello,

please add .scad (https://openscad.org) files with .cpp color syntax highlighting.

```
% mdls -name kMDItemContentType -name kMDItemContentTypeTree some-programatic-3d-description.scad
kMDItemContentType     = "dyn.ah62d4rv4ge81g25bqu"
kMDItemContentTypeTree = (
    "public.item",
    "dyn.ah62d4rv4ge81g25bqu",
    "public.data"
)
```

Thanks for your work :-) !