feat(cheddar): Use syntax highlighting assets from bat

This uses Nix to inject the path to the syntax highlighting assets
that ship with the bat source code into the cheddar build at compile
time, where the Rust compiler then inserts it into the binary via
macros.

bat has a lot of custom syntax highlighting definitions that they
collected from all over the place (including for languages like Nix!)
and this makes them accessible to cheddar.

Also if you're reading this, can you just take a moment to appreciate
how incredible it is that Nix just lets us do something like this?!

third_party/default.nix

@@ -22,6 +22,7 @@ let
     # of the repo. They become available under `pkgs.third_party.<name>`
     inherit (nixpkgs)
       bashInteractive
+      bat
       buildGoPackage
       cacert
       cachix