Add public TableView.BeginEditAsync(slot) to start an edit like F2 (#430)

src/TableViewCell.cs

@@ -234,9 +234,9 @@
 
     /// <summary>
     /// Begins a live resize-drag preview for this cell: generously (re)measures its content once so
     /// widening can freely reveal more of it, and creates this cell's own <see cref="Clip"/> geometry
     /// and gridline shift transform. These are per-cell instances (not shared across cells — WinUI
     /// throws if the same <see cref="RectangleGeometry"/> is assigned as <see cref="Clip"/> on more
     /// than one element at a time), mutated in place every frame by
     /// <see cref="UpdateResizePreviewClip"/>/<see cref="UpdateGridLineShift"/> — still no Measure/Arrange
     /// per frame, just not a single shared instance across every row.
@@ -502,6 +502,31 @@
         return false;
     }
 
+    /// <summary>
+    /// Begins editing this cell, as a double tap or F2 does.
+    /// </summary>
+    /// <remarks>
+    /// The control starts an edit session from <see cref="OnDoubleTapped"/> and from the
+    /// <c>TableView</c> key handler, but offers no public way to ask for one. A column whose RESTING
+    /// cell carries an affordance of its own - a drop-down arrow, a picker button - needs to open the
+    /// editor on a single click of that affordance, and that is not a gesture the control can infer.
+    /// <para>The same conditions apply as to a double tap: a read-only cell, a cell with no owning
+    /// <see cref="TableView"/>, a table that is already editing, or a column drawing itself through
+    /// <see cref="TableViewColumn.UseSingleElement"/> will not begin an edit, and this returns
+    /// <see langword="false"/>. <c>BeginningEdit</c> is raised, and a handler that cancels it is
+    /// honoured, exactly as on the existing paths.</para>
+    /// </remarks>
+    /// <returns><see langword="true"/> if the cell entered edit mode; otherwise <see langword="false"/>.</returns>
+    public bool BeginEdit()
+    {
+        if (IsReadOnly || TableView is null || TableView.IsEditing || Column?.UseSingleElement is not false)
+        {
+            return false;
+        }
+
+        return BeginCellEditing(new RoutedEventArgs());
+    }
+
     /// <summary>
     /// Prepares the cell for editing.
     /// </summary>