add new toggleTorch() method to platform interface

@@ -177,8 +177,6 @@ class MethodChannelMobileScanner extends MobileScannerPlatform {
     await methodChannel.invokeMethod<void>('resetScale');
   }
 
-  // TODO: remove the 'torch' function from native
-
   @override
   Future<void> setZoomScale(double zoomScale) async {
     await methodChannel.invokeMethod<void>('setScale', zoomScale);
@@ -275,6 +273,11 @@ class MethodChannelMobileScanner extends MobileScannerPlatform {
   }
 
   @override
+  Future<void> toggleTorch() async {
+    await methodChannel.invokeMethod<void>('toggleTorch');
+  }
+
+  @override
   Future<void> updateScanWindow(Rect? window) async {
     if (_textureId == null) {
       return;

@@ -90,6 +90,11 @@ abstract class MobileScannerPlatform extends PlatformInterface {
     throw UnimplementedError('stop() has not been implemented.');
   }
 
+  /// Toggle the torch on the active camera on or off.
+  Future<void> toggleTorch() {
+    throw UnimplementedError('toggleTorch() has not been implemented.');
+  }
+
   /// Update the scan window to the given [window] rectangle.
   ///
   /// Any barcodes that do not intersect with the given [window] will be ignored.

@@ -355,6 +355,14 @@ class MobileScannerWeb extends MobileScannerPlatform {
   }
 
   @override
+  Future<void> toggleTorch() {
+    throw UnsupportedError(
+      'Setting the torch state is not supported for video tracks on the web.\n'
+      'See https://developer.mozilla.org/en-US/docs/Web/API/MediaTrackConstraints#instance_properties_of_video_tracks',
+    );
+  }
+
+  @override
   Future<void> updateScanWindow(Rect? window) {
     // A scan window is not supported on the web,
     // because the scanner does not expose size information for the barcodes.