Add fish function for generating and saving completions

assets/default/fish/functions/update_completions.fish

@@ -0,0 +1,20 @@
+function update_completions --description "Update completions for a command" --argument-names "target_command"
+    # Usage: update_completions kubectl completion fish
+    #
+    # This generates new completions, sources them, and then saves it to your
+    # user completions directory for future loading.
+
+    if not set -q target_command ; or [ -z "$target_command" ]
+        echo "Must pass some command to generate completions. eg: update_completions kubectl completion fish" 1>&2
+        return 1
+    end
+
+    set --local completion_dir "$HOME/.config/fish/completions"
+    if set -q XDG_CONFIG_HOME
+        set --local completion_dir "$XDG_CONFIG_HOME"
+    end
+
+    set --local completion_path "$completion_dir/$target_command.fish"
+    eval $argv[1..] | tee "$completion_path" | source
+    echo "Completions for $target_command generated, sourced, and stored in $completion_path for future shells" 1>&2
+end