Sharp should soon be releasing AVIF support. This PR uses the alpha-3 version of libvips and upcoming version of Sharp to add support for avif images.

This should work the same as webp: there is an X-Avif header or `avif=1` param, and the detection is expected to be handled by a lamnda function the same as WebP (check for `accept:image/avif` header).

The results in `test` are good and bad:

```
┌───────────────┬───────────────┬────────────────────┬──────────┬──────────┬──────────┬───────────────┬───────────────┐
│ Image         │ Original Size │ Tachyon Size       │ 100px    │ 300px    │ 700px    │ 700px webp    │ 700px avif    │
├───────────────┼───────────────┼────────────────────┼──────────┼──────────┼──────────┼───────────────┼───────────────┤
│ briefing-cop… │ 442.52 KB     │ 112.67 KB (25%)    │ 3.01 KB  │ 9.34 KB  │ 28.51 KB │ 15.31 KB (53… │ 14.57 KB (51… │
│ hdr.jpg       │ 222.58 KB     │ 145.56 KB (65%)    │ 10.4 KB  │ 23.53 KB │ 85.59 KB │ 80.86 KB (94… │ 111.09 KB (1… │
│ icons.png     │ 94.24 KB      │ 27.37 KB (29%)     │ 3.89 KB  │ 11.31 KB │ 26.94 KB │ 23.93 KB (88… │ 13.25 KB (49… │
│ Website.png   │ 32.38 KB      │ 33.78 KB (104%)    │ 3.26 KB  │ 13.27 KB │ 33.78 KB │ 19.81 KB (58… │ 22.21 KB (65… │
│ humans.png    │ 2.22 MB       │ 853.21 KB (37%)    │ 8.96 KB  │ 54.66 KB │ 273.89 … │ 139.19 KB (5… │ 174.48 KB (6… │
└───────────────┴───────────────┴────────────────────┴──────────┴──────────┴──────────┴───────────────┴───────────────┘
```

In some cases it's less than webp, and in some cases more. I think it might be to do with these comparisons being quality:100, where we might do better at slightly dropping quality.