Feature request: Linkify code symbols in the model response

Given an LLM response that contains a code symbol (e.g. class name, method, etc.), we should link it to its exact location on GitHub. For instance, if you're chatting with Hugging Face's Transformers library:

• Original response: "To define a BERT model using the Hugging Face Transformers library, you can use the BertModel class."

• Link-ified response: "To define a BERT model using the Hugging Face Transformers library, you can use the BertModel class."

This should be done as a post-processing step in chat.py, once the model is done streaming.

Work items:

1. Produce an AST (Abstract Syntax Tree) for every file in the repository. This can be done via the tree_sitter library, which we're already using in chunk.py.
2. Identify class / method / file names in the model response, potentially using regular expressions.
3. Look up the strings matched by the regular expressions in the per-file ASTs.
4. Map the code symbol to a github URL.