fish: implement shellInitLast (after others)

Since the module system doesn't allow specifying order on types.lines
users can't specify anything to run after what modules have put into
shellInitInteractive since it runs last. This implements a fourth field
that runs after all others, not to be used by HM modules, but regular
users.

Co-authored-by: Carl Hjerpe <git@hjerpe.xyz>

modules/programs/fish.nix

@@ -314,6 +314,15 @@ in {
           initialisation.
         '';
       };
+
+      shellInitLast = mkOption {
+        type = types.lines;
+        default = "";
+        description = ''
+          Shell script code called during interactive fish shell
+          initialisation, this will be the last thing executed in fish startup.
+        '';
+      };
     };
 
     programs.fish.plugins = mkOption {
@@ -469,6 +478,8 @@ in {
           ${cfg.interactiveShellInit}
 
         end
+
+        ${cfg.shellInitLast}
       '';
     }
     {