stomach_size_multiplier for dragon size mutations (CleverRaven#62613)

Summary
Added stomach_size_multiplier in line with mainline size mutations to black dragon size mutations.

Purpose of change
Makes the black dragon mutation line viable considering the increased necessary nutrient intake.

Describe the solution
Added stomach_size_multiplier in line with mainline size mutations to black dragon size mutations.

Describe alternatives you've considered
Increasing metabolism_modifier and vitamins_absorb_multi, but that would both be illogical and go against the creators' intent with this mutation line.

Testing
drank water till full, confirmed stomach contents, emptied stomach. Repeated this with all 4 concerned mutations.

Additional context
Exact values are copied from mainline, can be considered placeholder until whoever is working on this decides what they should be.

data/mods/Magiclysm/mutations/mutations.json

@@ -227,6 +227,7 @@
     "category": [ "DRAGON_BLACK" ],
     "passive_mods": { "str_mod": 2 },
     "weight_capacity_modifier": 1.05,
+    "stomach_size_multiplier": 1.5,
     "anger_relations": [ [ "HUMAN", 10 ] ]
   },
   {
@@ -246,6 +247,7 @@
     "category": [ "DRAGON_BLACK" ],
     "passive_mods": { "str_mod": 2 },
     "weight_capacity_modifier": 1.05,
+    "stomach_size_multiplier": 1.5,
     "anger_relations": [ [ "HUMAN", 20 ] ]
   },
   {
@@ -270,6 +272,7 @@
     "restricts_gear": [ "torso", "leg_l", "leg_r", "arm_l", "arm_r", "hand_l", "hand_r", "head", "foot_l", "foot_r" ],
     "destroys_gear": true,
     "weight_capacity_modifier": 1.1,
+    "stomach_size_multiplier": 2.0,
     "anger_relations": [ [ "HUMAN", 30 ] ]
   },
   {
@@ -344,6 +347,7 @@
     "restricts_gear": [ "torso", "leg_l", "leg_r", "arm_l", "arm_r", "hand_l", "hand_r", "head", "foot_l", "foot_r" ],
     "destroys_gear": true,
     "weight_capacity_modifier": 1.1,
+    "stomach_size_multiplier": 2.0,
     "anger_relations": [ [ "HUMAN", 40 ] ]
   },
   {