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/CommandContextExtensions.cs

@@ -0,0 +1,19 @@
+using Remora.Discord.Commands.Contexts;
+using Remora.Discord.Commands.Extensions;
+using Remora.Rest.Core;
+
+namespace Octobot.Extensions;
+
+public static class CommandContextExtensions
+{
+    public static bool TryGetContextIDs(
+        this ICommandContext context, out Snowflake guildId,
+        out Snowflake channelId, out Snowflake executorId)
+    {
+        channelId = default;
+        executorId = default;
+        return context.TryGetGuildID(out guildId)
+               && context.TryGetChannelID(out channelId)
+               && context.TryGetUserID(out executorId);
+    }
+}