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/controllers/configuration/config_entry.dm

@@ -1,4 +1,6 @@
 /datum/config_entry
+	/// Do not instantiate if type matches this
+	abstract_type = /datum/config_entry
 	/// Read-only, this is determined by the last portion of the derived entry type
 	var/name
 	/// The configured value for this entry. This shouldn't be initialized in code, instead set default
@@ -13,8 +15,6 @@
 	var/deprecated_by
 	/// The /datum/config_entry type that supersedes this one
 	var/protection = NONE
-	/// Do not instantiate if type matches this
-	var/abstract_type = /datum/config_entry
 	/// Force validate and set on VV. VAS proccall guard will run regardless.
 	var/vv_VAS = TRUE
 	/// Controls if error is thrown when duplicate configuration values for this entry type are encountered