chore: move single-instance plugin

plugins/single-instance/src/platform_impl/macos.rs

@@ -0,0 +1,12 @@
+#![cfg(target_os = "macos")]
+
+use crate::SingleInstanceCallback;
+use tauri::{
+    plugin::{self, TauriPlugin},
+    Manager, Runtime,
+};
+pub fn init<R: Runtime>(f: Box<SingleInstanceCallback<R>>) -> TauriPlugin<R> {
+    plugin::Builder::new("single-instance").build()
+}
+
+pub fn destroy<R: Runtime, M: Manager<R>>(_manager: &M) {}