refactor: move modeltypes to own folder, move IDs to file of respective type

PluralKit.Core/Models/PKSystem.cs

@@ -4,7 +4,34 @@ using Newtonsoft.Json;
 
 using NodaTime;
 
+
+
 namespace PluralKit.Core {
+
+    public readonly struct SystemId: INumericId<SystemId, int>
+    {
+        public int Value { get; }
+
+        public SystemId(int value)
+        {
+            Value = value;
+        }
+
+        public bool Equals(SystemId other) => Value == other.Value;
+
+        public override bool Equals(object obj) => obj is SystemId other && Equals(other);
+
+        public override int GetHashCode() => Value;
+
+        public static bool operator ==(SystemId left, SystemId right) => left.Equals(right);
+
+        public static bool operator !=(SystemId left, SystemId right) => !left.Equals(right);
+
+        public int CompareTo(SystemId other) => Value.CompareTo(other.Value);
+
+        public override string ToString() => $"System #{Value}";
+    }
+
     public class PKSystem
     {
         // Additions here should be mirrored in SystemStore::Save