Register any LangChain chain as a magic with %ai register

[JasonWeill]

Problem

Users must write custom Python code in this repository to add a new LangChain chain or AI model.

Proposed Solution

Add a %ai register command that registers a LangChain chain with a new name.

Registered LangChain chains should show up when the user runs %ai list.

Modify the example notebooks and documentation to refer to this new command.