Add the possibility to enable the AutoFilter feature (#35)

See https://support.microsoft.com/en-us/office/use-autofilter-to-filter-your-data-7d87d63e-ebd0-424b-8106-e2ab61133d92

Author: 0xced

src/Simplexcel.TestApp/Program.cs

@@ -138,7 +138,7 @@ static void Main()
             populatedSheet.Populate(EnumeratePopulateTestData(), cacheTypeColumns: true);
             wb.Add(populatedSheet);
 
-            var frozenTopRowSheet = new Worksheet("Frozen Top Row");
+            var frozenTopRowSheet = new Worksheet("Frozen Top Row") { AutoFilter = true };
             frozenTopRowSheet.Cells[0, 0] = "Header 1";
             frozenTopRowSheet.Cells[0, 1] = "Header 2";
             frozenTopRowSheet.Cells[0, 2] = "Header 3";

src/Simplexcel/Worksheet.cs

@@ -94,6 +94,14 @@ public ColumnWidthCollection ColumnWidths
         /// </summary>
         public LargeNumberHandlingMode LargeNumberHandlingMode { get; set; }
 
+        /// <summary>
+        /// Whether to enable the AutoFilter feature on the first non-empty row.
+        /// <para/>
+        /// Defaults to <see langword="false"/>.
+        /// </summary>
+        /// <remarks>See <a href="https://support.microsoft.com/en-us/office/use-autofilter-to-filter-your-data-7d87d63e-ebd0-424b-8106-e2ab61133d92">Use AutoFilter to filter your data</a></remarks>
+        public bool AutoFilter { get; set; }
+
         /// <summary>
         /// Get the cell with the given cell reference, e.g. Get the cell "A1". May return NULL.
         /// </summary>

src/Simplexcel/XlsxInternal/XlsxWriter.cs

@@ -473,8 +473,10 @@ private static Relationship CreateSheetFile(Worksheet sheet, int sheetIndex, Rel
                 }
 
                 var sheetData = new XElement(Namespaces.workbook + "sheetData");
+                XlsxRow firstRow = null;
                 foreach (var row in rows.OrderBy(rk => rk.Key))
                 {
+                    firstRow ??= row.Value;
                     var re = new XElement(Namespaces.workbook + "row", new XAttribute("r", row.Value.RowIndex));
                     foreach (var cell in row.Value.Cells)
                     {
@@ -498,6 +500,15 @@ private static Relationship CreateSheetFile(Worksheet sheet, int sheetIndex, Rel
                 }
                 doc.Root.Add(sheetData);
 
+                var firstRowCells = firstRow?.Cells;
+                if (sheet.AutoFilter && firstRowCells?.Count > 0)
+                {
+                    var firstRef = firstRowCells.First().Reference;
+                    var lastRef = firstRowCells.Last().Reference;
+                    var autoFilter = new XElement(Namespaces.workbook + "autoFilter", new XAttribute("ref", $"{firstRef}:{lastRef}"));
+                    doc.Root.Add(autoFilter);
+                }
+
                 sheetRels = null;
                 var hyperlinks = sheet.Cells.Where(c => c.Value != null && !string.IsNullOrEmpty(c.Value.Hyperlink)).ToList();
                 if (hyperlinks.Count > 0)