Add information about available internal functions

[MkuuWaUjinga]

This PR addresses #5 and #26 by

1. generating function signatures and descriptions for internal target code functions.
2. adding this information to the migration prompt based on the internal file dependencies in the source repository

Design decisions

• Use an LLM to get function signatures instead of static code analysis as they should work with any language.
• Use prompts to guarantee JSON output format instead of OpenAI's function calling feature. This makes it LLM-vendor independent in case other models are supported in the future.

[joshpxyne]

Great work!!