Support `.nix` files

[jukremer]

.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"
)