feat(store): add $destroy action

# What's changed?

* add action to remove store from global storage (thus destroying it)

packages/vue-flow/src/composables/useVueFlow.ts

@@ -65,6 +65,9 @@ export class Storage {
       ...toRefs(reactiveState),
       emits,
       id,
+      $destroy: () => {
+        this.remove(id)
+      },
     }
 
     this.set(id, flow)

packages/vue-flow/src/types/store.ts

@@ -189,6 +189,9 @@ export interface Actions extends ViewportFunctions {
 
   /** reset state to defaults */
   $reset: () => void
+
+  /** remove store instance from global storage and destroy it (will invalidate effect scopes) */
+  $destroy: () => void
 }
 
 export interface Getters {