feat: preserve leading whitespace in INI keys

Implements #126 - Add support for indented keys (e.g., ' endpoint_url=value') - Preserve leading spaces/tabs in key names during parse/write cycles - Add whitespace parsing to handle key indentation - Maintain backward compatibility with existing INI files - Add tests for various indentation scenarios This enables support for configuration formats like AWS config files that use indented keys for hierarchical organization. Sorry for the PR noise. I've switched to another branch so I could run the tests in my own fork against the main branch. 3rd time is the charm!