Allow u/npc_spawn_monster to pick randomly from a group for count > 1

[Procyonae]

Summary

None

Purpose of change

Fixes #70812

Describe the solution

Adds a new field "single_type" that defaults to false but can be defined as true to replicate the old behaviour, I couldn't find any existing EoCs that want appear to want this to occur though (have a couple of ideas how it could be used tho)
Simplifies existing EoCs that were using the workaround of multiple 1 count EoCs
Updates documentation

Describe alternatives you've considered

Testing

Tested using the below EoCs with expected results

  {
    "type": "effect_on_condition",
    "id": "hallucination_swarm_testing",
    "effect": [
      { "u_spawn_monster": "", "hallucination_count": 20, "target_range": 50, "lifespan": [ "2 hours", "9 hours" ] }
    ]
  },
  {
    "type": "effect_on_condition",
    "id": "hallucination_swarm_testing_2",
    "effect": [
      { "u_spawn_monster": "", "hallucination_count": 20, "target_range": 50, "lifespan": [ "2 hours", "9 hours" ], "single_target": true }
    ]
  },
  {
    "type": "effect_on_condition",
    "id": "hallucination_swarm_testing_3",
    "effect": [
      { "u_spawn_monster": "GROUP_PET_DOGS", "real_count": 20, "group": true }
    ]
  },
  {
    "type": "effect_on_condition",
    "id": "hallucination_swarm_testing_4",
    "effect": [
      { "u_spawn_monster": "GROUP_PET_DOGS", "real_count": 20, "group": true, "single_target": true }
    ]
  }

If anyone can think of anything else I should test comment below

Additional context

[GuardianDll]

Looks good for me
Didn't know default can be used with math