Add TFHoliday_Summer (#2350)

Valve has updated this holiday this year to use 2025 dates (2025-07-16 to 2025-09-16) so semi-relevant to add

Author: FlaminSarge

extensions/tf2/holiday.cpp

@@ -148,15 +148,16 @@ void HolidayManager::OnPluginLoaded(IPlugin *plugin)
 	PopulateHolidayVar(pRuntime, "TFHoliday_Birthday");
 	PopulateHolidayVar(pRuntime, "TFHoliday_Halloween");
 	PopulateHolidayVar(pRuntime, "TFHoliday_Christmas");
-	PopulateHolidayVar(pRuntime, "TFHoliday_EndOfTheLine");
 	PopulateHolidayVar(pRuntime, "TFHoliday_CommunityUpdate");
+	PopulateHolidayVar(pRuntime, "TFHoliday_EndOfTheLine");
 	PopulateHolidayVar(pRuntime, "TFHoliday_ValentinesDay");
 	PopulateHolidayVar(pRuntime, "TFHoliday_MeetThePyro");
 	PopulateHolidayVar(pRuntime, "TFHoliday_FullMoon");
 	PopulateHolidayVar(pRuntime, "TFHoliday_HalloweenOrFullMoon");
 	PopulateHolidayVar(pRuntime, "TFHoliday_HalloweenOrFullMoonOrValentines");
 	PopulateHolidayVar(pRuntime, "TFHoliday_AprilFools");
 	PopulateHolidayVar(pRuntime, "TFHoliday_Soldier");
+	PopulateHolidayVar(pRuntime, "TFHoliday_Summer");
 }
 
 void HolidayManager::OnPluginUnloaded(IPlugin *plugin)

gamedata/sm-tf2.games.txt

@@ -222,6 +222,7 @@
 			"TFHoliday_HalloweenOrFullMoonOrValentines"	"10"
 			"TFHoliday_AprilFools"		"11"
 			"TFHoliday_Soldier"			"12"
+			"TFHoliday_Summer"			"13"
 		}
 	}
 }

plugins/include/tf2.inc

@@ -222,15 +222,16 @@ enum TFHoliday
 public const TFHoliday TFHoliday_Birthday;
 public const TFHoliday TFHoliday_Halloween;
 public const TFHoliday TFHoliday_Christmas;
-public const TFHoliday TFHoliday_EndOfTheLine;
 public const TFHoliday TFHoliday_CommunityUpdate;
+public const TFHoliday TFHoliday_EndOfTheLine;
 public const TFHoliday TFHoliday_ValentinesDay;
 public const TFHoliday TFHoliday_MeetThePyro;
 public const TFHoliday TFHoliday_FullMoon;
 public const TFHoliday TFHoliday_HalloweenOrFullMoon;
 public const TFHoliday TFHoliday_HalloweenOrFullMoonOrValentines;
 public const TFHoliday TFHoliday_AprilFools;
 public const TFHoliday TFHoliday_Soldier;
+public const TFHoliday TFHoliday_Summer;
 
 enum TFObjectType
 {