add openai embedding function to nodejs client (#107)

- openai is an optional dependency for lancedb
- added an example to show how to use it

node/examples/ts/package.json

@@ -17,6 +17,6 @@
     "typescript": "*"
   },
   "dependencies": {
-    "vectordb": "^0.1.0"
+    "vectordb": "file:../.."
   }
 }