Copy/pasting vector and raster content between other graphics apps

*This is a good first issue but not for beginners.*

We should be able to copy vector and raster content, including hierarchies of layers and groups, between creative programs. Both into Graphite and out of Graphite. This shouldn't break existing copy-pasting within Graphite, which needs to have actual graph information rather than just vector or bitmap layer info.

This will require research into how the clipboard APIs let us encode different types of data, and if perhaps we need a degraded experience on web due to API limitations compared to a full separate implementation for the Tuari desktop version. Testing other creative apps and using a utility for inspecting clipboard contents at a technical level will be a useful way to approach this. It's also worth checking if this differs between OSs.

Helpful reading: https://alexharri.com/blog/clipboard

首先研究链接文章中介绍的 Clipboard APIs，并使用合适的工具检查其他创意应用中的剪贴板内容。在保留 Graphite 现有内部图复制粘贴功能的同时，比较不同操作系统上的 Web 行为与 Tauri 桌面版。完成标准是：矢量和光栅内容（包括图层和组层级）可以导入 Graphite 并从 Graphite 导出，且不会破坏内部复制粘贴功能。

由索引模型根据 Issue 内容生成。