Add extension update folder

[rtm516]

Adds an update folder that can be used by extensions to update themselves on boot. Works a lot like the normal Spigot/Paper one.

Requires GeyserMC/languages#136 Merged

Also moves to using Files.list for finding jars as we only want to try and load them from the root extensions folder and not recursively look for them

[Konicai]

also please test what happens when there are multiple of the same extension in the update folder, or multiple in the extensions folder

[onebeastchris]

Duplicate extensions in the "normal" extensions folder will be caught & warned about when loading the extensions at the 3rd stage. As for duplicate update entries... they would currently just be all put in the extensions folder and result in the same duplicate extension warning

[Konicai]

Duplicate extensions in the "normal" extensions folder will be caught & warned about when loading the extensions at the 3rd stage

fair, but both could be replaced to ensure the new jar is the one that gets loaded

As for duplicate update entries... they would currently just be all put in the extensions folder and result in the same duplicate extension warning

would be nice to avoid

[onebeastchris]

Looks good, just some minor nitpicks

[onebeastchris]

ready for merge, just needs to be documented on the wiki