[google_fonts] Extract the config class to its own file

[Piinks]

Extract Config from google_fonts_all_parts.dart to google_fonts_config.dart. Rename Config to GoogleFontsConfig to avoid namespace clutter. Add a deprecated Config typedef for backward compatibility. Export GoogleFontsConfig and the deprecated Config in lib/google_fonts.dart. Bump package version to 8.2.0.

Unblocks #11433

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Code Review

This pull request extracts the Config class into its own file (google_fonts_config.dart) and renames it to GoogleFontsConfig. The original Config class is deprecated via a typedef pointing to GoogleFontsConfig. Additionally, the package version is bumped to 8.2.0 in pubspec.yaml and the CHANGELOG.md is updated. There are no review comments, and I have no feedback to provide.