refactor(core): use useOnInitHandler to emit paneReady event

packages/core/src/composables/useOnInitHandler.ts

@@ -0,0 +1,14 @@
+import { watch } from 'vue'
+import { useVueFlow } from './useVueFlow'
+
+export function useOnInitHandler() {
+  const vfInstance = useVueFlow()
+
+  watch(vfInstance.viewportInitialized, (isInitialized) => {
+    if (isInitialized) {
+      setTimeout(() => {
+        vfInstance.emits.paneReady(vfInstance)
+      }, 1)
+    }
+  })
+}