feat: implement flat_map

[rapidfsub]

• Add flat_map_ok and flat_map_error for convenience
  • Naming reference: https://developer.apple.com/documentation/swift/result/flatmaperror(_:)
• Update .gitignore with a template created by gitignore.io

Use case

- @impl Phoenix.LiveView
- def handle_event("save", %{"form" => form}, socket) do
-   socket.assigns.form
-   |> AshPhoenix.Form.validate(form)
-   |> AshPhoenix.Form.submit()
-   |> case do
-     {:ok, _result} -> {:noreply, push_navigate(socket, to: ~p"/")}
-     {:error, form} -> {:noreply, assign(socket, :form, form)}
-   end
- end
+ @impl Phoenix.LiveView
+ def handle_event("save", %{"form" => form}, socket) do
+   rail do
+     form = AshPhoenix.Form.validate(form, socket.assigns.form)
+     _result <- AshPhoenix.Form.submit(form)
+     {:noreply, push_navigate(socket, to: ~p"/")}
+   end
+   |> Rail.flat_map_error(fn form -> {:noreply, assign(socket, :form, form)} end)
+ end