add support for image/avif

[Glutamat42]

Added support for mime type 'image/avif'. Code was generated by chatgpt, but it's matching this: https://stackoverflow.com/a/68322450 and i verified it against two avif images (confirmed mime type of them with file --mime-type)

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[mosuem]

As this repo has moved to https://github.com/dart-lang/tools/tree/main/pkgs/mime, please refile the PR there. Thanks!