Include support for the AVIF image format

[lukefiretoss]

The current MIME type for AVIF images is

image/avif

.
​https://github.com/AOMediaCodec/av1-avif/issues/59

Google Chrome in version 85 added support for the AVIF image format and Mozilla Firefox and Safari from Apple will not be that far behind in supporting it.

The AVIF image format provides great file size and compression gains over JPG, PNG and WebP images.

[ayeshrajans]

I'm on Firefox 81 beta, and I already have avif support. +1 from me, I'll submit a patch if the maintainers agree :).

[lukefiretoss]

Mozilla Firefox
​https://www.mozilla.org/en-US/firefox/80.0/releasenotes/

Google Chrome
​https://www.chromestatus.com/feature/4905307790639104

[ayeshrajans]

I submitted a PR to PHPMailer, which apparently I includes a list of mine types. I suppose SimplePie will need an update too. After that, it will be all in core.
I don't think the image editors will be compatible with heic or avif, so adding avif/heic to those wouldn't work out well.