Use AsyncSequence

[sindresorhus]

When we can target macOS 12.

https://github.com/apple/swift-evolution/blob/main/proposals/0314-async-stream.md

We have a lot of asynchronicity and some of our code is hard to reason about. This change will make it both more readable and also let us get rid of Progress (NSProgress) which is very quirky and not well documented.

For example, this piece would benefit from using AsyncStream:

Gifski/Gifski/util.swift

Line 347 in 2fbfc10

func generateCGImagesAsynchronously(

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Apple provides it for us: https://developer.apple.com/documentation/avfoundation/avassetimagegenerator/3967579-images?changes=_2