externs.json: add persistent_cache_index_manager.ts (to be removed once the @internal tag is removed)

Author: dconeybe

packages/firestore/externs.json

@@ -31,6 +31,7 @@
     "packages/util/dist/src/environment.d.ts",
     "packages/util/dist/src/compat.d.ts",
     "packages/util/dist/src/obj.d.ts",
+    "packages/firestore/src/api/persistent_cache_index_manager.ts",
     "packages/firestore/src/protos/firestore_bundle_proto.ts",
     "packages/firestore/src/protos/firestore_proto_api.ts",
     "packages/firestore/src/util/error.ts",