[feat] `tauri add` should add single-instance's init method with a default closure

[FabianLars]

Describe the problem

currently if the user mindlessly runs tauri add single-instance (which happens a lot) they end up with something that doesn't compile.

Describe the solution you'd like

We should have special handling for this plugin and add a simple

.plugin(tauri_plugin_single_instance::init(|app, args, cwd| {}))

without a _ so that they actually get warnings about this.

Alternatives considered

No response

Additional context

No response