Add .nvmrc file to ensure node version

[markwitt1]

Using version 16 created this bug for me:

PineconeClient: Error calling upsert: TypeError: stream.getReader is not a function

[mayooear]

Thanks for this. There are a few issues with using .nvmrc including the added dependency on nvm which most users don't use, plus it may also add more friction to user local setup.

I have made a PR specifying the node.js version in the engines field of package.json