Support `.nix` files

`.nix` files are currently not supported. However, they are supported by `highlight`.

Outout by `mdls` command:

```
kMDItemContentType     = "dyn.ah62d4rv4ge8064p2"
kMDItemContentTypeTree = (
    "public.data",
    "public.item",
    "dyn.ah62d4rv4ge8064p2"
)
```