Add LangChain recipe

[dlqqq]

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[conda-forge-webservices]

Hi! This is the friendly automated conda-forge-linting service.

I just wanted to let you know that I linted all conda-recipes in your PR (recipes/langchain) and found it was in an excellent condition.

[dlqqq]

@hwchase17 This is the PR that stages LangChain to eventually be published on Conda Forge. 🎉 Trust me, I had to move some mountains to get this working. 😅

Next steps from your end:

• Agree to be listed as a maintainer for this package.
• (optional) Provide me a list of GitHub aliases you'd like to add as additional maintainers of this Conda Forge package.

[dlqqq]

Superseded by #22515.