Build a Pack or Expand node's ports from a struct variable

The port editor gets 'From variable' next to '+ Field': pick a struct variable
of the behavior and its fields land as ports. It only adds - a field the node
already declares stays as it is, because it may carry an edge - and the entry
is disabled once there is nothing left to add. A list of structs counts too:
it declares the shape of every item, which is exactly the bundle a Pack builds
for it. A color field arrives as a number port; the graph has no color type.

AddPortsCommand is new underneath: adding five ports had to be one undo step,
and it checks every key before it appends a single one, so a collision does
not leave half the list behind. AddPortCommand is a three-line subclass of it,
which leaves its sixteen call sites untouched.

src/editor/inspector/nodeSection.ts

@@ -53,7 +53,7 @@ export function buildNodeSection(host: InspectorHost, assetId: string, nodeId: s
   const problem = callProblem(node, behaviorLookup(host.state.project), assetId);
   if (problem) addNote(host, problem, 'warning');
   if (declaresVariablePorts(node.type)) buildVariablePortSection(host, assetId, node, asset.variables);
-  else if (def.declaresPorts) buildPortSection(host, assetId, node);
+  else if (def.declaresPorts) buildPortSection(host, assetId, node, asset.variables);
 
   if (def.params.length > 0) addTitle(host, 'SETTINGS');
   for (const param of def.params) {