Adds two heavy midround rulesets, mass traitors and mass changelings (#93447)

## About The Pull Request

- [ ] I tested this pr

Adds "Midround Mass Traitors" and "Midround Mass Changelings" heavy
rulesets

Midround Mass Traitors will turn a number of the crew into sleeper
agents

Midround Mass Lings will spawn a number of changeling meteors 

BY DEFAULT:
- Traitors will spawn 2-4 sleeper agents
- Lings will spawn 2-3 changeling meteors
- They won't run without >=2 candidates
- They have no config blacklist (so the HoP can roll mass midround
traitor, despite not being able to roll normal midround traitor. However
default blacklisted roles like security officers are still blocked)
- They're weighted impossible on tier 0, same weight as most midrounds
on tier 1, and fairly high on tiers 2 and 3
- They're repeatable but very unlikely to repeat
- Significantly higher min pop brackets than their solo counter part

## Why It's Good For The Game

Adds more options for lategame antags than big midrounds

## Changelog

🆑 Melbert
add: Two new heavy dynamic rulesets: "Midround Mass Traitors" and
"Midround Mass Changelings"
/🆑

code/modules/client/preferences/middleware/antags.dm

@@ -149,11 +149,12 @@ GLOBAL_LIST_INIT(non_ruleset_antagonists, list(
 	for (var/datum/dynamic_ruleset/ruleset as anything in subtypesof(/datum/dynamic_ruleset))
 		var/datum/antagonist/antagonist_type = initial(ruleset.preview_antag_datum)
 		var/antag_flag = initial(ruleset.pref_flag)
-		if (isnull(antagonist_type) || isnull(antag_flag))
+		if(isnull(antagonist_type) || isnull(antag_flag))
 			continue
 
-		// antag_flag is guaranteed to be unique by unit tests.
-		antagonists[initial(ruleset.pref_flag)] = antagonist_type
+		// antag_flag is guaranteed to be unique for all non-RULESET_VARIATION rulesets
+		// the ||= covers specifically those rulesets - prefer the first one over variations
+		antagonists[initial(ruleset.pref_flag)] ||= antagonist_type
 
 	var/list/generated_icons = list()