Implement AVIF Image Format Support

This PR introduces support for the AVIF image format in the SwiftGD library. The addition of AVIF format capabilities enhances the library's versatility, aligning it with modern image processing standards.

Changes

AVIF Formatter Implementation: Added AVIFFormatter structures, conforming to LibGdImportableFormatter and LibGdExportableFormatter, to handle AVIF image import and export.
Enum Extensions: Updated ImportableFormat and ExportableFormat enums to include the AVIF format, allowing for seamless integration of AVIF handling in the existing SwiftGD workflow.
Testing and Validation: Conducted tests of AVIF images to ensure proper functionality and integration with the existing codebase.

Motivation

AVIF, as a modern and efficient image format, is increasingly being adopted for its superior compression and image quality. Integrating AVIF support into SwiftGD ensures that the library remains up-to-date with current image processing trends and technologies.

Impact

This addition provides SwiftGD users the ability to work with AVIF images, extending the library's applicability to a broader range of projects and use cases. It also ensures compatibility with web and application development standards that are increasingly incorporating AVIF.

Looking forward to your feedback and suggestions for any further improvements.

twostraws / SwiftGD