Performance issue solved

When printing tens of thousands of lines, there is a performance issue. The TableRowCollection -> Add method was being called millions of times.
Once  it is called, it returns the index  of the added element.
As the element is added to a List (new List<TableRow>, I believe it is not needed to use IndexOf because it will be added always at the last position.
Replacing the IndexOf call by a Count - 1 improves the performance in this scenario.
Related issue: spectreconsole#975

src/Spectre.Console/Widgets/Table/TableRowCollection.cs

@@ -58,7 +58,7 @@ public int Add(IEnumerable<IRenderable> columns)
  {
  var row = CreateRow(columns);
  _list.Add(row);
- return _list.IndexOf(row);
+ return _list.Count - 1;
  }
  }