Sync-viewer Packages

`@cesium-extends/sync-viewer` is an npm package for synchronizing two Cesium Viewer instances. [中文 Readme](./README_CN.md)

`@nsc-earth-2/sync-viewer` is an npm package for synchronizing two Cesium Viewer instances. [中文 Readme](./README_CN.md)

`@nsc-earth-v2/sync-viewer` is an npm package for synchronizing two Cesium Viewer instances. [中文 Readme](./README_CN.md)