An icon to show before the input field and outside of the decoration's container.
The size and color of the icon is configured automatically using an IconTheme and therefore does not need to be explicitly given in the icon widget.
The trailing edge of the icon is padded by 16dps.
The decoration's container is the area which is filled if filled is
true and bordered per the border. It's the area adjacent to
decoration.icon and above the widgets that contain helperText,
errorText, and counterText.
final Widget icon