moves abstract_type up to datum, spawners wont spawn them (#92909)

## About The Pull Request
moves all implementations (im aware of) for "Im a parent type dont spawn
me please" to the datum layer to standardized behavior
adds a standerized proc for filtering out "bad" items that we dont want
spawning. applies to it the subtype vendor, gifts, and a new spawner and
mystery box for a random gun (neither playerfacing)
"port" of https://github.com/shiptest-ss13/Shiptest/pull/4621



https://github.com/user-attachments/assets/22f6f0b2-b44e-411a-b3dc-6b97dc0287aa

small warning: I dont have EVERY abstract type defined right now but,
ive done a good enough job for now. Im tired of data entry rn
## Why It's Good For The Game
standardizing behavior. Might be a micro hit to performance however

having this lets us not rely on icon state to determine whether
something is a parent type and makes it much easier to tell something is
a parent type (could be applied further to things like admin spawning
menus and things like that).

need feedback on if this is actually good for the game.
## Changelog
🆑
add: Soda cans show up in the silver slime drink table.
add: Examine tag for items that are not mean to show up ingame.
refactor: Standardizes how gifts rule out abstract types.
fix: gifts no longer check if something has an inhand, massively
expanding the list of potential items.
/🆑

code/modules/loadout/loadout_items.dm

@@ -30,6 +30,8 @@ GLOBAL_LIST_INIT(all_loadout_categories, init_loadout_categories())
  * Singleton that holds all the information about each loadout items, and how to equip them.
  */
 /datum/loadout_item
+	/// The abstract parent of this loadout item, to determine which items to not instantiate
+	abstract_type = /datum/loadout_item
 	/// The category of the loadout item. Set automatically in New
 	VAR_FINAL/datum/loadout_category/category
 	/// Displayed name of the loadout item.
@@ -49,8 +51,6 @@ GLOBAL_LIST_INIT(all_loadout_categories, init_loadout_categories())
 	/// Whether this item can be reskinned.
 	/// Only works if the item has a "unique reskin" list set.
 	var/can_be_reskinned = FALSE
-	/// The abstract parent of this loadout item, to determine which items to not instantiate
-	var/abstract_type = /datum/loadout_item
 	/// The actual item path of the loadout item.
 	var/obj/item/item_path
 	/// Icon file (DMI) for the UI to use for preview icons.