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

[ShGKme]

Problem

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

Proposal 1: make scopeKey public

nextcloud-browser-storage/lib/scopedstorage.ts

Lines 19 to 21 in 8379a79

	private scopeKey(key: string) {
	return `${this.scope}${key}`
	}

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`
}