Lots more work on implementing battling

PkmnLib.Static/Utils/EnumerableHelpers.cs

@@ -1,7 +1,13 @@
 namespace PkmnLib.Static.Utils;
 
+/// <summary>
+/// Extension methods for <see cref="IEnumerable{T}"/>.
+/// </summary>
 public static class EnumerableHelpers
 {
+    /// <summary>
+    /// Returns all elements of the enumerable that are not null.
+    /// </summary>
     public static IEnumerable<T> WhereNotNull<T>(this IEnumerable<T?> enumerable) where T : class =>
         enumerable.Where(x => x is not null)!;
 }