Fix links to localhost (#157)

Closes #156

Author: reakaleek

src/Elastic.Markdown/IO/ConfigurationFile.cs

@@ -23,7 +23,7 @@ public record ConfigurationFile : DocumentationFile
 	public HashSet<string> Files { get; } = new(StringComparer.OrdinalIgnoreCase);
 	public HashSet<string> ImplicitFolders { get; } = new(StringComparer.OrdinalIgnoreCase);
 	public Glob[] Globs { get; } = [];
-	public HashSet<string> ExternalLinkHosts { get; } = new(StringComparer.OrdinalIgnoreCase) { "elastic.co", "github.com", };
+	public HashSet<string> ExternalLinkHosts { get; } = new(StringComparer.OrdinalIgnoreCase) { "elastic.co", "github.com", "localhost", };
 
 	private readonly Dictionary<string, string> _substitutions = new(StringComparer.OrdinalIgnoreCase);
 	public IReadOnlyDictionary<string, string> Substitutions => _substitutions;

src/Elastic.Markdown/Myst/InlineParsers/DiagnosticLinkInlineParser.cs

@@ -60,7 +60,7 @@ public override bool Match(InlineProcessor processor, ref StringSlice slice)
 
 		if (Uri.TryCreate(url, UriKind.Absolute, out var uri) && uri.Scheme.StartsWith("http"))
 		{
-			var baseDomain = string.Join('.', uri.Host.Split('.')[^2..]);
+			var baseDomain = uri.Host == "localhost" ? "localhost" : string.Join('.', uri.Host.Split('.')[^2..]);
 			if (!context.Configuration.ExternalLinkHosts.Contains(baseDomain))
 			{
 				processor.EmitWarning(