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/UserExtensions.cs

@@ -0,0 +1,11 @@
+using Remora.Discord.API.Abstractions.Objects;
+
+namespace Octobot.Extensions;
+
+public static class UserExtensions
+{
+    public static string GetTag(this IUser user)
+    {
+        return user.Discriminator is 0000 ? $"@{user.Username}" : $"{user.Username}#{user.Discriminator:0000}";
+    }
+}