assets: Add Mac folder icon
assets: Add Mac folder icon

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GitHub-Pull-Request: https://github.com/syncthing/syncthing/pull/4238

AudriusButkevicius/syncthing,syncthing/syncthing,syncthing/syncthing,wweich/syncthing,uok/syncthing,rumpelsepp/syncthing,AudriusButkevicius/syncthing,wweich/syncthing,rumpelsepp/syncthing,lkwg82/syncthing,rumpelsepp/syncthing,imsodin/syncthing,imsodin/syncthing,syncthing/syncthing,xduugu/syncthing,uok/syncthing,imsodin/syncthing,uok/syncthing,syncthing/syncthing,lkwg82/syncthing,AudriusButkevicius/syncthing,syncthing/syncthing,rumpelsepp/syncthing,imsodin/syncthing,syncthing/syncthing,jryans/syncthing,xduugu/syncthing,lkwg82/syncthing,jryans/syncthing,wweich/syncthing,jryans/syncthing,jryans/syncthing,AudriusButkevicius/syncthing,xduugu/syncthing,xduugu/syncthing,wweich/syncthing,imsodin/syncthing,lkwg82/syncthing,uok/syncthing