Added missing function - old fish

fish_cfg/functions/fish_is_root_user.fish

@@ -0,0 +1,12 @@
+# Defined in /usr/share/fish/functions/fish_is_root_user.fish @ line 4
+function fish_is_root_user --description 'Check if the user is root'
+    if test "$EUID" = 0 2>/dev/null
+        return 0
+    end
+
+    if contains -- $USER root toor Administrator
+        return 0
+    end
+
+    return 1
+end