Add 'sub-graphs' to ComfyUI

### Feature Idea

Node-based programs (like Geogen, WorldMachine, etc.) feature what are called "sub-graphs" or "Macros".

Double-clicking a subgraph/Macro node will open a blank node graph.

In this new graph/space, one can place a series of nodes; and connect then to special Input & Output nodes.

The Subgraph/Macro nodes in the main graph/space, update their inputs & outputs according to the number of connected Input & Output nodes within the sub-graph.

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The common purpose of this feature; is to clean up the main graph/space.

e.g. I aim to use such, to hide a bunch of nodes that the user(myself) doesn't need to see.

There are other purposes for subgraphs;

1. In WorldMachine, Macros are treated like Custom Nodes; and are distributed on the forums like such.

2. Saving and loading sub-graphs into workflows is a better alterative to that of ComfyUI's 'Templates'.

### Existing Solutions

_No response_

### Other

_No response_