Fix exception when running on the web

[dsyrstad]

When you run the example/lib/main.dart on the web, you get:

======== Exception caught by widgets library =======================================================
The following UnsupportedError was thrown building DragTarget<FlexDragTargetData>(dirty, state: _DragTargetState<FlexDragTargetData>#800e2):
Unsupported operation: Platform._operatingSystem

The relevant error-causing widget was: 
  DragTarget<FlexDragTargetData> DragTarget:file:///home/dsyrstad/IdeaProjects/appflowy-board/lib/src/widgets/reorder_flex/drag_target.dart:127:25
When the exception was thrown, this was the stack: 
dart-sdk/lib/_internal/js_dev_runtime/private/ddc_runtime/errors.dart 294:3       throw_
dart-sdk/lib/_internal/js_dev_runtime/patch/io_patch.dart 244:5                   _operatingSystem
dart-sdk/lib/io/platform_impl.dart 56:40                                          get operatingSystem
dart-sdk/lib/io/platform.dart 83:44                                               get operatingSystem
dart-sdk/lib/_internal/js_dev_runtime/private/ddc_runtime/operations.dart 1430:8  get
dart-sdk/lib/io/platform.dart 161:46                                              get isIOS
dart-sdk/lib/_internal/js_dev_runtime/private/ddc_runtime/operations.dart 1430:8  get
packages/appflowy_board/src/widgets/reorder_flex/drag_target.dart 201:21          [_buildDraggableWidget]

This is due to drag_target.dart blindly checking the Platform, which doesn't work on web. This PR fixes the issue by checking for kIsWeb first.

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