fix: Fix from-index resolution with nab 0.0.16

[kemzeb]

Attempts to fix #697.

Current approach is to bring over the nab package that holds the original API we were using that was moved to this package.

[kemzeb]

Making it a draft PR, since this is a first for me in using a coding agent and want more time to review it (exciting stuff so far). But I also want to show one approach to this problem.

I'm not sure if we should use nab since it's the CLI utility. Then again it does expose nab.config.model as public API. Since were looking to fully support [tool.nab] in pyproject.toml this would be the easiest solution. Alternatively, we could see what else we could do with the existing libraries. This may involve us having to maintain our own logic for parsing [tool.nab].