Add subgraph skeleton classes (#997)

Allows downstream consumers to use subgraph types ahead of impl.

src/subgraph/SubgraphOutputNode.ts

@@ -0,0 +1,12 @@
+import type { Positionable } from "@/interfaces"
+import type { NodeId } from "@/LGraphNode"
+
+import { SubgraphIONodeBase } from "./SubgraphIONodeBase"
+
+export class SubgraphOutputNode extends SubgraphIONodeBase implements Positionable {
+  readonly id: NodeId = -20
+
+  get slots() {
+    return this.subgraph.outputs
+  }
+}