[Contribution] PocketFlow-Nim: Nim Implementation

Hello again!

Further to the Zig version, I've written a Nim implementation of PocketFlow - https://github.com/bkataru/PocketFlow-Nim

Like before, the Nim module is a library that implements the core abstractions (Node, Flow, Context) following the same patterns as the other language implementations. It includes unit tests, documentation, and a working example using Ollama for LLM integration. I've built it taking reference from the Rust, Go, TypeScript, and Zig ports.

I can make a PR to add Nim to the README listing of available language versions. Furthermore, if you're okay with this as the official Nim implementation, could you transfer this repo to the @The-Pocket org?