watchOnce 
一度だけトリガーされる`watch`。
使用方法 
コールバック関数が一度トリガーされると、watchは自動的に停止します。
ts
import { watchOnce } from '@vueuse/core'
watchOnce(source, () => {
  // triggers only once
  console.log('source changed!')
})型定義 
typescript
export declare function watchOnce<
  T extends Readonly<WatchSource<unknown>[]>,
  Immediate extends Readonly<boolean> = false,
>(
  source: [...T],
  cb: WatchCallback<MapSources<T>, MapOldSources<T, Immediate>>,
  options?: WatchOptions<Immediate>,
): WatchStopHandle
export declare function watchOnce<
  T,
  Immediate extends Readonly<boolean> = false,
>(
  sources: WatchSource<T>,
  cb: WatchCallback<T, Immediate extends true ? T | undefined : T>,
  options?: WatchOptions<Immediate>,
): WatchStopHandle