fix(ts-sdk): make natural an optional peer so pg is not installed transitively

natural@8 hard-depends on pg ^8.18.0, mongoose, redis and memjs. As a
required peer it was auto-installed for every mem0ai user, so pg stayed in
the tree even after pg itself became optional. lemmatizeForBm25 already
falls back to a built-in stemmer when natural is missing.

mem0-ts/package.json

@@ -263,6 +263,9 @@
     },
     "@types/pg": {
       "optional": true
+    },
+    "natural": {
+      "optional": true
     }
   },
   "engines": {