Setting StringExtensions as internal

src/Discord.Net.Core/Extensions/StringExtensions.cs

@@ -2,7 +2,7 @@
 
 namespace Discord
 {
-    public static class StringExtensions
+    internal static class StringExtensions
     {
         public static bool IsNullOrUri(this string url) =>
             string.IsNullOrEmpty(url) || Uri.IsWellFormedUriString(url, UriKind.Absolute);