Split extension methods into separate classes (#161)

This PR splits the extension methods contained in `Extensions.cs` into
separate classes in the `Octobot.Extensions` namespace. This was done
for multiple reasons:
1) The `Extensions.cs` violates SRP (Single Responsibility Principle) -
it takes upon itself every extension method for many types
2) Having a separate class for each extended type is a standard practice
- take a look at
[Remora.Discord](https://github.com/Remora/Remora.Discord/tree/main/Backend/Remora.Discord.Rest/Extensions)
or [osu!](https://github.com/ppy/osu/tree/master/osu.Game/Extensions)
3) Having all extension methods in one file makes it hard to find the
method you want

src/Extensions/CollectionExtensions.cs

@@ -0,0 +1,40 @@
+using Remora.Results;
+
+namespace Octobot.Extensions;
+
+public static class CollectionExtensions
+{
+    public static TResult? MaxOrDefault<TSource, TResult>(
+        this IEnumerable<TSource> source, Func<TSource, TResult> selector)
+    {
+        var list = source.ToList();
+        return list.Any() ? list.Max(selector) : default;
+    }
+
+    public static void AddIfFailed(this List<Result> list, Result result)
+    {
+        if (!result.IsSuccess)
+        {
+            list.Add(result);
+        }
+    }
+
+    /// <summary>
+    ///     Return an appropriate result for a list of failed results. The list must only contain failed results.
+    /// </summary>
+    /// <param name="list">The list of failed results.</param>
+    /// <returns>
+    ///     A successful result if the list is empty, the only Result in the list, or <see cref="AggregateError" />
+    ///     containing all results from the list.
+    /// </returns>
+    /// <exception cref="InvalidOperationException"></exception>
+    public static Result AggregateErrors(this List<Result> list)
+    {
+        return list.Count switch
+        {
+            0 => Result.FromSuccess(),
+            1 => list[0],
+            _ => new AggregateError(list.Cast<IResult>().ToArray())
+        };
+    }
+}