Sets the tabbing array for the buffer.
The tabs list determines the tabbing array. The tabbing array specifies the x-locations where each tab occurs. Tabs beyond the last specified tab are separated by a fixed amount tab-width. If in-units? is not #f, then tabs are specified in canvas units; otherwise, they are specified as a number of spaces. (If tabs are specified in spaces, then the graphic tab positions will change with the font used for the tab.)