feat: add custom font icons support

[P0labrD]

Why

Inspired by Golden toolkit font_loader.dart use of FontManifest.json that references all fonts by packages in build/unit_test/assets folder

Use loadAppFonts instead of loadFonts or loadFontsFromPackage

Todo

• [x] add usage examples
• [x] update README usage
• [ ] update README goldens screenshots on README