guix graph: Export node types and <node-type> accessors.

* guix/scripts/graph.scm (%node-types, node-type, node-type?,
  node-type-identifier, node-type-label, node-type-edges,
  node-type-convert, node-type-name, node-type-description): Export.

guix/scripts/graph.scm

@@ -39,6 +39,16 @@ (define-module (guix scripts graph)
             %bag-emerged-node-type
             %derivation-node-type
             %reference-node-type
+            %node-types
+
+            node-type
+            node-type?
+            node-type-identifier
+            node-type-label
+            node-type-edges
+            node-type-convert
+            node-type-name
+            node-type-description
 
             %graphviz-backend
             graph-backend?