feat: export piniaSymbol

Author: xiongjie

packages/pinia/src/index.ts

@@ -1,7 +1,7 @@
 /**
  * @module pinia
  */
-export { setActivePinia, getActivePinia } from './rootStore'
+export { setActivePinia, getActivePinia, piniaSymbol } from './rootStore'
 export { createPinia, disposePinia } from './createPinia'
 export type { Pinia, PiniaPlugin, PiniaPluginContext } from './rootStore'