[Request]: Support tracking all battery aspects

### Plugin

battery_plus

### Use case

At the moment, there is no option to track changes in battery level or any other aspect. However, for many apps, it would be quite reasonable to react to such changes, by displaying a snackbar message or sending analytics data.

iOS provides support for this functionality:
[Documentation for level notification](https://developer.apple.com/documentation/uikit/uidevice/batteryleveldidchangenotification?language=objc)
[Documentation for power save mode notification](https://developer.apple.com/documentation/foundation/nsprocessinfopowerstatedidchangenotification)

```swift
Publishers.Merge3(
NotificationCenter.default.publisher(for: UIDevice.batteryLevelDidChangeNotification),
NotificationCenter.default.publisher(for: UIDevice.batteryStateDidChangeNotification),
NotificationCenter.default.publisher(for: NSNotification.Name.NSProcessInfoPowerStateDidChange)
)
.sink { _ in self.onBatteryInfoChanged() }
```

Android also supports monitoring battery level changes:
[Documentation](https://developer.android.com/training/monitoring-device-state/battery-monitoring)
[Power save mode changed notification](https://developer.android.com/reference/android/os/PowerManager#ACTION_POWER_SAVE_MODE_CHANGED)

```kotlin
applicationContext?.registerReceiver(
this,
IntentFilter().apply {
addAction(Intent.ACTION_BATTERY_CHANGED)
addAction(Intent.ACTION_BATTERY_LOW)
addAction(PowerManager.ACTION_POWER_SAVE_MODE_CHANGED)
}
)
```

I’m not entirely sure about other platforms, but I would expect similar functionality to be available.

### Proposal

```dart
class BatteryInfo {
final double level;
final bool isLow;
final bool isInBatterySaveMode;
final BatteryState state;
}

battery.onBatteryInfoChanged.listen((BatteryInfo info) {
// Do something with new info
});
```

I actually already have an implementation for iOS and Android and would be happy to contribute if this proposal is considered useful.

@mhadaily cc

Überprüfe das battery_plus-Plugin und die im Issue verlinkten iOS- und Android-Benachrichtigungs-APIs, beginnend mit der vorhandenen iOS- und Android-Implementierung, auf die der Autor verweist. Bestätige die unterstützten Plattformen und BatteryInfo-Felder und verifiziere anschließend, dass Änderungen an Batteriestand, Batteriestatus und Energiesparmodus über den vorgeschlagenen Stream bereitgestellt werden.

Vom Indexierungsmodell aus dem Issue-Text verfasst.