Detect HEIC/HEIF format by checking header bytes

[CORDEA]

Hi, thank you to this useful package and its maintainers 🙌

HEIC/HEIF format is already supported in #58, but I want to enhance this detection by checking the header bytes. I have referred to the following documents.

• https://www.loc.gov/preservation/digital/formats/fdd/fdd000526.shtml
• https://www.iana.org/assignments/media-types/image/heic
• https://nokiatech.github.io/heif/technical.html

The header bytes look like this:

• image/heif
  • 0x0, 0x0, 0x0, 0x18, 0x66, 0x74, 0x79, 0x70, 0x6d, 0x69, 0x66, 0x31, 0x0, ...
  • ... f, t, y, p, m, i, f, 1 ...
• image/heic
  • 0x0, 0x0, 0x0, 0x18, 0x66, 0x74, 0x79, 0x70, 0x68, 0x65, 0x69, 0x63, 0x0, ...
  • ... f, t, y, p, h, e, i, c ...

Also, with this and #58, I think dart-lang/tools#401 can be closed.

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[devoncarew]

Thanks for the update to the changelog and the tests!