Fish, simplify functions, move n alias call

Simplifies the git_ family of functions to make fish_right_prompt more
readable. git_prompt function is added, which prints the relevant git
prompt information. The git_ family of functions is moved into the
git_prompt function, and fish_right_prompt calls git_prompt

Going forward I'm trying to have modules in charge of the data for
other modules, so I moved the nnn alias call from fish to n, and added a
handy "extraFunctions" config option to fish. Maintaining the fish
module's control of the functions, but allowing others to add stuff in

hmModules/apps/file-browsers/nnn/default.nix

@@ -13,5 +13,8 @@
       NNN_FIFO = "/tmp/nnn.fifo";
       NNN_TRASH = lib.mkIf config.trash.enable "1";
     };
+    fish.extraFunctions = lib.mkIf config.fish.enable {
+      n = ''${builtins.readFile ./nnn_fish_function.fish}'';
+    };
   };
 }