Add TypeScript types for lib

**Problem**
When working with config, the error messages could be more descriptive.

**Suggestion**
Improve error handling and add detailed error messages.

**Context**
This would help with debugging and maintenance.

The issue mentions the lib and config handling but names no files, tests, or entry points. First clarify whether the intended work is adding TypeScript types or improving configuration error messages; only then identify the relevant lib code and define a testable completion condition.

Written by the indexing model from the issue text.