refactor: Cleaner distinguishing of def-name node in onNodeClick

This was made possible by recent changes to make `PrimerNode` a proper discriminated union.

src/App.tsx

@@ -248,7 +248,7 @@ const AppNoError = ({
         <TreeReactFlow
           {...(selection && { selection })}
           onNodeClick={(_e, node) => {
-            if (!("nodeType" in node.data)) {
+            if (node.type == "primer-def-name") {
               setSelection({
                 def: node.data.def,
               });