Migrate to type-safe model ID structs

PluralKit.Core/Models/INumericId.cs

@@ -0,0 +1,11 @@
+using System;
+
+namespace PluralKit.Core
+{
+    public interface INumericId<T, out TInner>: IEquatable<T>, IComparable<T>
+        where T: INumericId<T, TInner>
+        where TInner: IEquatable<TInner>, IComparable<TInner>
+    {
+        public TInner Value { get; }
+    }
+}