add stub for the web implementation

-export 'src/web/base.dart';
-export 'src/web/jsqr.dart';
-export 'src/web/zxing.dart';

+import 'package:flutter_web_plugins/flutter_web_plugins.dart';
+import 'package:mobile_scanner/src/mobile_scanner_platform_interface.dart';
+
+/// A web implementation of the MobileScannerPlatform of the MobileScanner plugin.
+class MobileScannerWeb extends MobileScannerPlatform {
+  /// Constructs a [MobileScannerWeb] instance.
+  MobileScannerWeb();
+
+  static void registerWith(Registrar registrar) {
+    MobileScannerPlatform.instance = MobileScannerWeb();
+  }
+}

@@ -44,4 +44,4 @@ flutter:
         pluginClass: MobileScannerPlugin
       web:
         pluginClass: MobileScannerWebPlugin
-        fileName: mobile_scanner_web_plugin.dart
+        fileName: web/mobile_scanner_web.dart