[Feature Request] Provide a way to listen `storage` event

Problem

There is no clean way to listen for a specific BrowserStorage instance storage event

Proposal 1: make scopeKey public

https://github.com/nextcloud-libraries/nextcloud-browser-storage/blob/8379a79b62109c08f57f3498edb028e126961015/lib/scopedstorage.ts#L19-L21

Providing it as a public property allows filtering the storage events manually to handle only a specific browserStorage instance.

Proposal 2: provide a method to subscribe for the storage event

Option 1: add onStorage method

class ScopedStorage implements NextcloudStorage {
  onStorage(key: string, callback) {
  onStorage(callback) {
    const callbackWithFilter = () => {...}

    window.addEventListener('storage', callbackWithFilter)

    return () => window.removeEventListener('storage', callbackWithFilter)
  }

  offStorage(callback) {
    // callback should be stored in a Map
  }
}

Option 2: provide additional functions

declare function onBrowserStorageChange(browserStorage: ScopedStorage, key?: string, callback)
declare function onBrowserStorageChange(browserStorage: ScopedStorage, callback)

Option 3: extend EventTarget

class ScopedStorage implements NextcloudStorage extends EventTarget {
  // Override `addEventListener` and `removeEventListener`
}

Start with lib/scopedstorage.ts, especially the scopeKey definition referenced in the issue, and inspect how ScopedStorage relates to the browser storage event. Compare the proposed public property, subscription methods, and EventTarget approaches, then determine which API the project should adopt and how callers would unsubscribe or filter by key. Done means a decided, documented way to listen for changes from one ScopedStorage instance.