Bundled subgraph fixes (#4964)

### Group support for subgraph unpacking
The unpacking code would silently delete groups (the cosmetic colored
rectangles). They are now correctly transferred.
### Fix subgraph node position on conversion to subgraph
Converting to subgraph will no longer cause nodes to inch upwards

![subgraph-conversion-positioning](https://github.com/user-attachments/assets/e120c3f9-5602-4dba-9075-c1eadb534f9a)
### Make unpacking use same positioning calcs as conversion
Non trivial, but unpacking is now a proper inverse for conversion.

![subgraph-conversion-inverse](https://github.com/user-attachments/assets/4fcaffca-1c97-4d71-93f7-1af569b1c941)
### Clean up old output links when unpacking
Unpacked nodes were left with dangling outputs. This would cause
cascading issues later, such as when consecutively unpacking nested
subgraphs.
### Minor refactoring for code clarity

┆Issue is synchronized with this [Notion
page](https://www.notion.so/PR-4964-Bundled-subgraph-fixes-24e6d73d365081d3a043ef1531d9d38a)
by [Unito](https://www.unito.io)

src/lib/litegraph/test/subgraph/SubgraphConversion.test.ts

@@ -3,6 +3,7 @@ import { assert, describe, expect, it } from 'vitest'
 import {
   ISlotType,
   LGraph,
+  LGraphGroup,
   LGraphNode,
   LiteGraph
 } from '@/lib/litegraph/src/litegraph'
@@ -80,7 +81,7 @@ describe('SubgraphConversion', () => {
       expect(graph.nodes.length).toBe(4)
       expect(graph.links.size).toBe(2)
     })
-    it('Should keep reroutes', () => {
+    it('Should keep reroutes and groups', () => {
       const subgraph = createTestSubgraph({
         outputs: [{ name: 'value', type: 'number' }]
       })
@@ -98,6 +99,7 @@ describe('SubgraphConversion', () => {
       const outer = createNode(graph, ['number'])
       const outerLink = subgraphNode.connect(0, outer, 0)
       assert(outerLink)
+      subgraph.add(new LGraphGroup())
 
       subgraph.createReroute([10, 10], innerLink)
       graph.createReroute([10, 10], outerLink)
@@ -105,6 +107,7 @@ describe('SubgraphConversion', () => {
       graph.unpackSubgraph(subgraphNode)
 
       expect(graph.reroutes.size).toBe(2)
+      expect(graph.groups.length).toBe(1)
     })
     it('Should map reroutes onto split outputs', () => {
       const subgraph = createTestSubgraph({