highlight.js supports syntax highlighting .nix files, but the Gerrit
diff components doesn't map the .nix mimetype onto the nix language.
.nix appears to already be taken by another mimetype that isn't
Nix-related, but we just map that onto nix anyway.
https://usercontent.irccloud-cdn.com/file/AVhoPvrb/highlighted.png
Change-Id: I842b29c78355e5bec580e711e25d693284ab6f59
Reviewed-on: https://cl.tvl.fyi/c/depot/+/253
Reviewed-by: tazjin <mail@tazj.in>
Reviewed-by: q3k <q3k@q3k.org>
