add base for the new barcode reader

+import 'dart:async';
+import 'dart:js_interop';
+
+import 'package:js/js.dart';
+import 'package:mobile_scanner/src/enums/mobile_scanner_error_code.dart';
+import 'package:mobile_scanner/src/enums/torch_state.dart';
+import 'package:mobile_scanner/src/mobile_scanner_exception.dart';
+import 'package:web/web.dart';
+
+/// This class represents the base interface for a barcode reader implementation.
+abstract class BarcodeReader {
+  const BarcodeReader();
+
+  /// The id for the script tag that loads the barcode library.
+  ///
+  /// If a script tag with this id already exists,
+  /// the library will not be loaded again.
+  String get scriptId => 'mobile-scanner-barcode-reader';
+
+  /// The script url for the barcode library.
+  String get scriptUrl;
+
+  /// Load the barcode reader library.
+  ///
+  /// Does nothing if the library is already loaded.
+  Future<void> maybeLoadLibrary() async {
+    // Script already exists.
+    if (document.querySelector('script#$scriptId') != null) {
+      return;
+    }
+
+    final Completer<void> completer = Completer();
+
+    final HTMLScriptElement script = (document.createElement('script') as HTMLScriptElement)
+      ..id = scriptId
+      ..async = true
+      ..defer = false
+      ..type = 'application/javascript'
+      ..lang = 'javascript'
+      ..crossOrigin = 'anonymous'
+      ..src = scriptUrl
+      ..onload = allowInterop((JSAny _) {
+        if (!completer.isCompleted) {
+          completer.complete();
+        }
+      }).toJS;
+
+    script.onerror = allowInterop((JSAny _) {
+      if (!completer.isCompleted) {
+        // Remove the script if it did not load.
+        document.head!.removeChild(script);
+
+        completer.completeError(
+          const MobileScannerException(
+            errorCode: MobileScannerErrorCode.genericError,
+            errorDetails: MobileScannerErrorDetails(
+              message: 'Could not load the BarcodeReader script due to a network error.',
+            ),
+          ),
+        );
+      }
+    }).toJS;
+
+    document.head!.appendChild(script);
+
+    await completer.future;
+  }
+
+  /// Set the flashlight state for the device.
+  Future<void> setTorchState(TorchState torchState) {
+    throw UnimplementedError('setTorchState() has not been implemented.');
+  }
+
+  /// Stop the barcode reader and dispose of the video stream.
+  Future<void> stop() {
+    throw UnimplementedError('stop() has not been implemented.');
+  }
+}