Node Subgraph Editing

[adamgerhant]

Closes #1172

Double-clicking nodes (except those based on proto nodes) enters their subgraph, replacing what's currently viewed in the node graph.

Todo

• [x] A breadcrumb trail shows which node you're in relative to the document's root network
• [ ] Determine input/output type from compiled node network in order to display nodes without document definition. (Complete but blocked by #1756)
• [x] Remove "Imports" field for node network, and instead store import index in each NodeInput::Network. This allows multiple nodes to use the same import field, and enables creation of the "Imports" node
• [ ] Create a UI only "Imports" node, used for customizing the network import index for any node
• [ ] Create a UI only "Exports" node, used for customizing the exports of a node
• [ ] Save root node for each network, which is an Option<NodeId> default node that DocumentMetadata will use to display/update/add layers
• [ ] Add toggle for displaying WasmEditorAPI import and links in Document network