Add StringBuilder & Markdown extensions (#206)

In this PR, I have added StringBuilder extensions to avoid `.Append`
reuse such as `.Append("- ").AppendLine()`

Closes #205

---------

Signed-off-by: mctaylors <cantsendmails@mctaylors.ru>

src/Extensions/MarkdownExtensions.cs

@@ -0,0 +1,16 @@
+namespace Octobot.Extensions;
+
+public static class MarkdownExtensions
+{
+    /// <summary>
+    /// Formats a string to use Markdown Bullet formatting.
+    /// </summary>
+    /// <param name="text">The input text to format.</param>
+    /// <returns>
+    /// A markdown-formatted bullet string.
+    /// </returns>
+    public static string BulletPoint(string text)
+    {
+        return $"- {text}";
+    }
+}