Add comparator for .dmg files (MacOS disk image)

.dmg files are MACOSX installer files, comparing them result in binary diff, examples:

Below files return for file zlib compressed data https://github.com/audacity/audacity/releases/download/Audacity-3.0.0/audacity-macos-3.0.0.dmg https://github.com/audacity/audacity/releases/download/Audacity-3.0.2/audacity-macos-3.0.2.dmg

https://github.com/maculateConception/aural-player/releases/download/2.12.1/AuralPlayer-2.12.1.dmg https://github.com/maculateConception/aural-player/releases/download/2.12.0/AuralPlayer-2.12.0.dmg

Below files return for file lzfse encoded, lzvn compressed https://github.com/BlueWallet/BlueWallet/releases/download/v6.0.2/BlueWallet.6.0.2.dmg https://github.com/BlueWallet/BlueWallet/releases/download/v6.0.1/BlueWallet.6.0.1.dmg

.dmg files can be sometimes extracted by: (the .dmg files vary in various details such as compression used & type, etc...)

1. 7z x filename.dmg from p7zip-full package (in above links work only on AuralPlayer app).
2. dmg2img filename.dmg filename.img command from dmg2img package convert the .dmg file to .img file, but the result is still file which diffoscope don't recognize and thus binary diff is shown, file for them return data or: DOS/MBR boot sector; partition 1 : ID=0xee, start-CHS (0x3ff,254,63), end-CHS (0x3ff,254,63), startsector 1, 105122 sectors, extended partition table (last)
3. using unhfs command from hfsexplorer: https://github.com/unsound/hfsexplorer/releases/ https://stackoverflow.com/questions/59960134/need-command-line-tools-to-extract-binaries-from-dmg-file-on-osx
4. mount the image: sudo mount -t hfsplus filename.dmg /mnt (type hfs or hfsplus)

More info & other tools: https://en.wikipedia.org/wiki/Apple_Disk_Image https://askubuntu.com/questions/38112/how-can-i-open-a-dmg-file