Default to link portals both ways

[benwoo1110]

Since this plugin primarily is used to link nether and end portal to mimick vanilla behaviour, Maybe when running something like /mvnp link nether overworld netherworld should link both nether portal from overworld to netherworld and netherworld to overworld, so we don't have to run 2 commands.

Some people may still want to have individual linking so we shouldn't break existing behaviour as well. I think we can solve this by having an argument for it, or a slightly different command like /mvnp duallink for this new 2 way linking.