Support for custom Dart source directories beyond hardcoded 'lib' folder

The package currently hardcodes the search for Dart files to the lib directory only.

This creates limitations for projects that don't follow the standard single-package Flutter structure where

• Localization files are shared between multiple apps/packages.
• Dart code exists in directories other than lib
• The localization package is separate from consuming applications

In these cases, the tool will incorrectly identify localization keys as "unused".

Proposed Solution: Add configuration options to specify custom directories to scan.