Refactor isVersionInfo() to use json` module

src/util.ts

@@ -634,16 +634,16 @@ interface PersistedVersionInfo {
 }
 
 function isVersionInfo(x: unknown): x is VersionInfo {
-  const candidate = x as Partial<VersionInfo> | null;
   return (
-    typeof candidate === "object" &&
+    json.isObject(x) &&
-    candidate !== null &&
+    json.validateSchema(
-    typeof candidate.version === "string" &&
+      {
-    (candidate.features === undefined ||
+        version: json.string,
-      (typeof candidate.features === "object" &&
+        features: json.undefinable(json.object),
-        candidate.features !== null)) &&
+        overlayVersion: json.undefinable(json.number),
-    (candidate.overlayVersion === undefined ||
+      },
-      typeof candidate.overlayVersion === "number")
+      x,
+    )
   );
 }