Support nix-eval fish function

Wrapping the `nix eval` incantation in a fish function for two reasons:
1. Document that this is how to evaluate Nix from a file.
2. Provide a more ergonomic way of evaluating Nix from a file.

configs/shared/.config/fish/config.fish

@@ -1,6 +1,5 @@
 # While I work use a variety of programs, below of some of my more commonly used
-# programs that I have decided to support with aliases.
-# Applications
+# programs that I have decided to support with aliases. # Applications
 #   java:       jv
 #   tmux:       t
 #   $EDITOR:    e
@@ -76,6 +75,13 @@ function fish_prompt
     echo -e "\n$suffix "
 end
 
+function nix-eval --description 'Evaluate Nix expression from file, $1.'
+    # Notice the empty string at the end of the invocation here. This is
+    # intentional. For more information, see this issue:
+    # https://github.com/NixOS/nix/issues/2078
+    nix eval --file $argv[1] ""
+end
+
 # Setup fzf for fuzzily finding commands, files, directories
 source (fzf-share)/key-bindings.fish && fzf_key_bindings