chore: move single-instance plugin

plugins/single-instance/examples/vanilla/src-tauri/src/main.rs

@@ -0,0 +1,13 @@
+#![cfg_attr(
+  all(not(debug_assertions), target_os = "windows"),
+  windows_subsystem = "windows"
+)]
+
+fn main() {
+  tauri::Builder::default()
+    .plugin(tauri_plugin_single_instance::init(|app, argv, cwd| {
+      println!("{}, {argv:?}, {cwd}", app.package_info().name);
+    }))
+    .run(tauri::generate_context!())
+    .expect("error while running tauri application");
+}