Immersive aether portals

Important note: Aether for 1.21.1 has native Immersive Portals support since version 1.5.1, so this datapack is no longer needed.

Makes Aether portal immersive, so you can see another side of the portal through it.

Works on both Sinytra Connector and native (Neo)Forge versions of Immersive Portals, and on Fabric from 1.21.1, since Aether was ported to fabric. Obviously need Aether mod.

Need to disable creation of native aether portals in `.minecraft\config\aether-server.toml on 1.20.4 and newer versions, or directly in savefolder in .minecraft\saves\_save_folder_name_\serverconfig\aether-server.toml on earlier versions. Set "Disables Aether Portal creation" = true in [Modpack]` section.

If you want to use dimension stacking with Aether, you also need to set `"Disables falling into the Overworld" = true in [Modpack]` section.