Add env_nested_max_split setting

[gsakkis]

New setting to limit the parsing of nested env vars split by env_nested_delimiter up to a given nesting depth.

The main use case is to allow env_nested_delimiter='_' to work with fields that contain underscores, as proposed here.

[hyperlint-ai]

The style guide flagged several spelling errors that seemed like false positives. We skipped posting inline suggestions for the following words:

• [Ss]ubstring

[hramezani]

Thanks @gsakkis for this PR.

I left some comments.