DataTable: highlight both the row label and column label when cursor type is 'cell'

[need47]

Currently when cursor type is cell, only the cell is highlighted. I found it often very helpful to highlight the corresponding row label and column label as well. Like below:

I achieved it with a small modification to [_should_highlight()](https://github.com/Textualize/textual/blob/main/src/textual/widgets/_data_table.py#L2478)

if type_of_cursor == "cell":
    return (
        cursor == target_cell
        or (target_cell.row == -1 and target_cell.column == cursor.column)     # <-- highlight column label
        or (target_cell.column == -1 and target_cell.row == cursor.row)        # <-- highlight row label
    )

The above changes also hightlight row/column label when mouse over a cell with a different color.

If you think this feature makes sense, I can create a pull request.