Add Persian text utilities to StringUtil class

Added RemoveDiacritics method to remove diacritical marks from Persian characters.
Added NormalizePersianNumbers method to convert Arabic numbers to Persian numbers.
Added IsPersianLetter method to check if a character is a Persian letter.

PersianDate/Utills/StringUtil.cs

@@ -94,4 +94,66 @@ public static string ToTitleCase(string input)
     {
         return CultureInfo.CurrentCulture.TextInfo.ToTitleCase(input.ToLower());
     }
+    /// <summary>
+    /// Removes diacritical marks from Persian characters.
+    /// <example>
+    /// <code>
+    /// string normalizedString = StringUtil.RemoveDiacritics("مَدرَسِه"); // Returns "مدرسه"
+    /// </code>
+    /// </example>
+    /// </summary>
+    /// <param name="input">The input string.</param>
+    /// <returns>The string without diacritical marks.</returns>
+    public static string RemoveDiacritics(string input)
+    {
+        string[] diacritics = { "\u064B", "\u064C", "\u064D", "\u064E", "\u064F", "\u0650", "\u0651", "\u0652" };
+        foreach (string diacritic in diacritics)
+        {
+            input = input.Replace(diacritic, string.Empty);
+        }
+        return input;
+    }
+
+    /// <summary>
+    /// Converts Arabic numbers to Persian numbers in a string.
+    /// <example>
+    /// <code>
+    /// string persianNumbers = StringUtil.NormalizePersianNumbers("123٤٥٦"); // Returns "۱۲۳۴۵۶"
+    /// </code>
+    /// </example>
+    /// </summary>
+    /// <param name="input">The input string.</param>
+    /// <returns>The string with Persian numbers.</returns>
+    public static string NormalizePersianNumbers(string input)
+    {
+        return input
+            .Replace('0', '۰')
+            .Replace('1', '۱')
+            .Replace('2', '۲')
+            .Replace('3', '۳')
+            .Replace('4', '۴')
+            .Replace('5', '۵')
+            .Replace('6', '۶')
+            .Replace('7', '۷')
+            .Replace('8', '۸')
+            .Replace('9', '۹')
+            .Replace('٤', '۴')
+            .Replace('٥', '۵')
+            .Replace('٦', '۶');
+    }
+
+    /// <summary>
+    /// Checks if a character is a Persian letter.
+    /// <example>
+    /// <code>
+    /// bool isPersian = StringUtil.IsPersianLetter('م'); // Returns true
+    /// </code>
+    /// </example>
+    /// </summary>
+    /// <param name="input">The input character.</param>
+    /// <returns>True if the character is a Persian letter; otherwise, false.</returns>
+    public static bool IsPersianLetter(char input)
+    {
+        return input is >= '\u0600' and <= '\u06FF';
+    }
 }