Add super basic group model/command

PluralKit.Core/Models/GroupId.cs

@@ -0,0 +1,26 @@
+namespace PluralKit.Core
+{
+    public readonly struct GroupId: INumericId<GroupId, int>
+    {
+        public int Value { get; }
+
+        public GroupId(int value)
+        {
+            Value = value;
+        }
+
+        public bool Equals(GroupId other) => Value == other.Value;
+
+        public override bool Equals(object obj) => obj is GroupId other && Equals(other);
+
+        public override int GetHashCode() => Value;
+
+        public static bool operator ==(GroupId left, GroupId right) => left.Equals(right);
+
+        public static bool operator !=(GroupId left, GroupId right) => !left.Equals(right);
+
+        public int CompareTo(GroupId other) => Value.CompareTo(other.Value);
+        
+        public override string ToString() => $"Member #{Value}";
+    }
+}