Added an example showing how to use Gemini with LangChain.js in Node

[pcoet]

Description of the change

Added an example showing how to use Gemini models with LangChain.js in Node.js.

Motivation

LangChain is a popular framework for developing LLM apps. This quickstart is a runnable example that illustrates some of the functionality discussed in the existing LangChain.js docs:

• LangChain.js: Google
• LangChain.js: ChatGoogleGenerativeAI

Note: Eventually I'd like to add a TypeScript version of this example, and perhaps also a web version. I might also add a small server implementation to make it a more typical Node app.

Type of change

Choose one: (Bug fix | Feature request | Documentation | Other): Other (example)

Checklist

• [x ] I have performed a self-review of my code.
• [x ] I have added detailed comments to my code where applicable.
• [x ] I have verified that my change does not break existing code.
• [x ] My PR is based on the latest changes of the main branch (if unsure, please run git pull --rebase upstream main).
• [x ] I am familiar with the Google Style Guide for the language I have coded in.
• [x ] I have read through the Contributing Guide and signed the Contributor License Agreement.