Support ImageMagick v7

[ReagentX]

Per the docs:

The "magick" command is the new primary command of the Shell API, replacing the old "convert" command.

imessage-exporter does not support this at the moment, but it should:

imessage-exporter/imessage-exporter/src/app/converter.rs

Lines 33 to 45 in 915cd1a

	impl Converter {
	/// Determine the converter type for the current shell environment
	pub fn determine() -> Option<Converter> {
	if exists("sips") {
	return Some(Converter::Sips);
	}
	if exists("convert") {
	return Some(Converter::Imagemagick);
	}
	eprintln!("No HEIC converter found, attachments will not be converted!");
	None
	}
	}