Turbopack: store ChunkingType in single-module-graph (#73837)
Based on https://github.com/vercel/next.js/pull/73572
- Model ChunkingType with Petgraph edges instead of Nodes
- For now, I removed the `SingleModuleGraphNode::Chunk` variant again. I don't need it right now and I'm not convinced that all the other usecases apart from chunking should have to constantly deal with that enum variant. And it's the Single **Module** Graph after all 🤷♂️
But we can of course still add it if we need it once we need it
No measureable performance impact
