Item editor ranged, magazine and ammo configuration

[snipercup]

This requires #45 to work. I will publish this PR as a draft to make sure #45 gets merged first.

Changes nothing gameplay-wise, but it allows you to edit items and give them properties for ranged weapons, magazine and ammo.

• In the item editor, adds the ranged, magazine and ammo tabs
• When the item is saved, the properties from the respective tabs is saved to the item json
• When the item is loaded into the item editor, the corresponding tab will be visible, allowing you to make edits
• I copied these properties from item_manager.gd. Their function will be decided later
• Adds a 9mm pistol and m4a1 rifle as items. They both use the same ammo for now
• Using this implementation means items do not have their own type, but if an item has the 'ranged' key, that means it's a ranged weapon. We use that instead of explicitly defining a type.

Here you can see the 'ranged' type is selected and the 'ranged' tab is visible

You can then switch over to the ranged tab:

When pressing the save button, it will be saved as