add stub for ZXing barcode reader

+import 'package:mobile_scanner/src/enums/barcode_format.dart';
+import 'package:mobile_scanner/src/web/barcode_reader.dart';
+import 'package:mobile_scanner/src/web/zxing/zxing_browser_multi_format_reader.dart';
+
+/// A barcode reader implementation that uses the ZXing library.
+final class ZXingBarcodeReader extends BarcodeReader {
+  ZXingBarcodeReader();
+
+  /// The internal barcode reader.
+  ZXingBrowserMultiFormatReader? _reader;
+
+  @override
+  String get scriptUrl => 'https://unpkg.com/@zxing/library@0.19.1';
+
+  /// Get the barcode format from the ZXing library, for the given [format].
+  int getZXingBarcodeFormat(BarcodeFormat format) {
+    switch (format) {
+      case BarcodeFormat.aztec:
+        return 0;
+      case BarcodeFormat.codabar:
+        return 1;
+      case BarcodeFormat.code39:
+        return 2;
+      case BarcodeFormat.code93:
+        return 3;
+      case BarcodeFormat.code128:
+        return 4;
+      case BarcodeFormat.dataMatrix:
+        return 5;
+      case BarcodeFormat.ean8:
+        return 6;
+      case BarcodeFormat.ean13:
+        return 7;
+      case BarcodeFormat.itf:
+        return 8;
+      case BarcodeFormat.pdf417:
+        return 10;
+      case BarcodeFormat.qrCode:
+        return 11;
+      case BarcodeFormat.upcA:
+        return 14;
+      case BarcodeFormat.upcE:
+        return 15;
+      case BarcodeFormat.unknown:
+      case BarcodeFormat.all:
+        return -1;
+    }
+  }
+}