Add NPC worldIndex to NPC update packets

[charles-m-knox]

NPC chat messages and health bars (and possibly other things) currently seem to only show on the first NPC in the client's list of tracked NPC's within player view. This PR fixes that:

• Updates NPC update packet handling to include a new worldIndex byte pair

Because this changes the packet structure, older clients won't work after this change if they're using the my code in runejs/server#433.

Code quality note: If there are any suggestions on a good way to refactor the lines of code that I repeated multiple times in src/main/java/org/runejs/client/media/renderable/actor/Npc.java, please feel free to suggest it.

See accompanying PR for the client: runejs/server#433

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edit: previously I coupled a few other changes into this PR, but since this is probably an (understandably) undesirable change, I'm going to remove the other changes into other future PRs

[Promises]

@charles-m-knox yeah updating the prebuild might be a good idea :D

[charles-m-knox]

@charles-m-knox yeah updating the prebuild might be a good idea :D

As discussed in Discord, I've added a commit to update the readme and removed the prebuilt dir in favor of using releases with attached jar files. This PR is all good from my end 👍 This will be addressed elsewhere since I've now updated this PR to only be concerned with the packet change.