Upgrade linked icon support

[jimathy]

Due to issues with other scripts (including my own), I made it a bit more dynamic.

• Detects wether an icon is an external link (nui:// or http://) and ignores changing it
• If the item image is a link already, ignore it
• If the icon is just an item name, create a nui link to grab the image
• Moved the "nui://" icon addition from the js file to the main.lua file
• Added the "update core" event to help with the script finding new items

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Fixes QoL issues with icons being added from links

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