This allows launching an offline instance with --offline --name=OfflineName. This is useful for playing split screen by creating two side-by-side instances, which is impossible online, unless one is using two separate paid accounts.
With this PR, it makes it possible to launch from a script - otherwise, one has to launch manually, which is a pain, or create offline profiles for each instance, which interferes with some functionality like skins (my autistic son takes great issue with his skin not being visible, when using offline profiles!).
Implementation is based on MultiMC, which supports this feature.
See also https://github.com/PrismLauncher/PrismLauncher/issues/1059 for discussion.
Signed-off-by: Michael Tyson <michael@atastypixel.com>
