Core: migrate item links out of main

[alwaysintreble]

What is this fixing or adding?

Moves the item linking out of main and into a multiworld method. also adds a test that the item links option parses correctly. the test could be pretty easily expanded but i couldn't think of anything useful without using a world object, though it might be worth creating a base world object for unit testing in the future.

How was this tested?

Unit test. it passes with #2911

If this makes graphical changes, please attach screenshots.