feat: add string.Slugify() extension

Generic slug generation was hand-rolled in a consuming project; upstreaming
it here so any project can share it instead of duplicating the logic.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

Author: dkorecko

Devity.Extensions/StringExtensions.cs

@@ -1,9 +1,27 @@
 using System.Text;
+using System.Text.RegularExpressions;
 
 namespace Devity.Extensions
 {
     public static class StringExtensions
     {
+        private static readonly Regex NonAlphaNumeric = new("[^a-z0-9]+", RegexOptions.Compiled);
+
+        /// <summary>
+        /// Converts a string into a lowercase slug joined by the given separator (e.g. "Hello World!" with
+        /// separator '-' becomes "hello-world"). Runs of non-alphanumeric characters collapse to a single
+        /// separator; leading/trailing separators are trimmed. Returns an empty string for blank input.
+        /// </summary>
+        public static string Slugify(this string str, char separator = '-')
+        {
+            if (string.IsNullOrWhiteSpace(str))
+                return string.Empty;
+
+            var lowered = str.Trim().ToLowerInvariant();
+            var collapsed = NonAlphaNumeric.Replace(lowered, separator.ToString());
+            return collapsed.Trim(separator);
+        }
+
         /// <summary>
         /// Will shorten the provided string object to the maximum of the provided maximum length. If string is shorter, full string is returned.
         /// </summary>