Icon-layout axis for <cngx-tab-group>, orthogonal to both skin and
orientation. Positions the *cngxTabIcon slot relative to the tab
label: 'start' (default) places the icon before the label in a row;
'end' places it after the label in a row; 'top' stacks the icon
above the label in a column; 'only' hides the label visually while
keeping it in the DOM as the accessible name.
Redirected purely via
the [data-icon-layout] host attribute.