fix: update asset loader

[juan-campuzano]

resolve #29

Font loading and registration improvements

• Updated loadAppFonts in asset_loader.dart to register each font under every possible alias (both the raw family name and any packages/<pkg>/<family> variants), ensuring icons render correctly in golden tests regardless of how IconData is constructed. [1] [2]
• Introduced the fontFamilyAliases helper, which derives all aliases a font should be registered under based on its manifest entry. This logic is now used in loadAppFonts and is tested extensively. [1] [2]

Documentation and test updates

• Improved documentation for loadAppFonts, fontFamilyAliases, and the IconExtension.convertToGolden method to clarify usage and indicate that manual conversion is now rarely needed. [1] [2]
• Added comprehensive unit tests for fontFamilyAliases to verify that all font aliasing scenarios are handled correctly.

Backwards compatibility

• Marked IconExtension.convertToGolden as deprecated, recommending use of loadAppFonts instead, but retained it for backwards compatibility and rare edge cases.