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

PluralKit.Core/Models/PKMember.cs

@@ -5,7 +5,33 @@ using Newtonsoft.Json;
 using NodaTime;
 using NodaTime.Text;
 
+
+
 namespace PluralKit.Core {
+    public readonly struct MemberId: INumericId<MemberId, int>
+    {
+        public int Value { get; }
+
+        public MemberId(int value)
+        {
+            Value = value;
+        }
+
+        public bool Equals(MemberId other) => Value == other.Value;
+
+        public override bool Equals(object obj) => obj is MemberId other && Equals(other);
+
+        public override int GetHashCode() => Value;
+
+        public static bool operator ==(MemberId left, MemberId right) => left.Equals(right);
+
+        public static bool operator !=(MemberId left, MemberId right) => !left.Equals(right);
+
+        public int CompareTo(MemberId other) => Value.CompareTo(other.Value);
+        
+        public override string ToString() => $"Member #{Value}";
+    }
+
     public class PKMember
     {
         // Dapper *can* figure out mapping to getter-only properties, but this doesn't work