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/StringExtensions.cs

@@ -0,0 +1,52 @@
+using System.Net;
+using Remora.Discord.Extensions.Formatting;
+
+namespace Octobot.Extensions;
+
+public static class StringExtensions
+{
+    /// <summary>
+    ///     Sanitizes a string for use in <see cref="Markdown.BlockCode(string)" /> by inserting zero-width spaces in between
+    ///     symbols used to format the string with block code.
+    /// </summary>
+    /// <param name="s">The string to sanitize.</param>
+    /// <returns>The sanitized string that can be safely used in <see cref="Markdown.BlockCode(string)" />.</returns>
+    private static string SanitizeForBlockCode(this string s)
+    {
+        return s.Replace("```", "​`​`​`​");
+    }
+
+    /// <summary>
+    ///     Sanitizes a string (see <see cref="SanitizeForBlockCode" />) and formats the string to use Markdown Block Code
+    ///     formatting with a specified
+    ///     language for syntax highlighting.
+    /// </summary>
+    /// <param name="s">The string to sanitize and format.</param>
+    /// <param name="language"></param>
+    /// <returns>
+    ///     The sanitized string formatted to use Markdown Block Code with a specified
+    ///     language for syntax highlighting.
+    /// </returns>
+    public static string InBlockCode(this string s, string language = "")
+    {
+        s = s.SanitizeForBlockCode();
+        return
+            $"```{language}\n{s.SanitizeForBlockCode()}{(s.EndsWith("`", StringComparison.Ordinal) || string.IsNullOrWhiteSpace(s) ? " " : "")}```";
+    }
+
+    public static string Localized(this string key)
+    {
+        return Messages.ResourceManager.GetString(key, Messages.Culture) ?? key;
+    }
+
+    /// <summary>
+    ///     Encodes a string to allow its transmission in request headers.
+    /// </summary>
+    /// <remarks>Used when encountering "Request headers must contain only ASCII characters".</remarks>
+    /// <param name="s">The string to encode.</param>
+    /// <returns>An encoded string with spaces kept intact.</returns>
+    public static string EncodeHeader(this string s)
+    {
+        return WebUtility.UrlEncode(s).Replace('+', ' ');
+    }
+}