Good Evening,
Simply replace the switch with a dimmer switch. Some places this is allowed in others it is not. It also could be considered an improper repair as you normally do not have to get a permit to change a switch.
Triacs make either good fuses or are on all the time when they fail. Make the assumption that if somebody plugs something in that they shouldn’t you get to replace the dimmer. You might want to buy a few spares. If the dimmer fails shorted the current limit is the unit plugged into the outlet, no more then that can flow assuming it is wired properly. I have bunches wired that way. Also note everybody stated the bottom plug is always on the top is controlled (a pseudo standard maybe). That indicates that a two conductor wire can go from the plug to the switch to effect the control (this is not always true).
Have Fun
Gil Shultz