This is called a proximity sensor. There are lots of them, some made for exactly this purpose (lining up a car in the garage).
I don’t remember seeing one that can connect out of the box with smartthings, though.
The most common way to get smartthings integration would be to add one to an Arduino and then probably use ST_anything to act on the report. But I remember a couple years ago there was somebody who set one up with a Z wave integration. I’ll see if I can find that one for you.
Meanwhile, check this out:
[DEPRECATED] ST_Anything - Arduino/ESP8266/ESP32
And here’s an Arduino-based project from one community member. The project report is really old, but the people in the ST anything link above could help you figure out what needs to be updated.
Smart Parking Spotter by Todd Wackford
The other alternative would be a pressure mat that would report if a car was on the mat. These are commonly used for driveway alarms and they are pretty easy to connect up to smart things. The problem, though, is getting things lined up just right, and that might be trickier.
Here’s another old project report based on the pressure mat. Again, some of this would have to be updated for the current platform but it should definitely be doable. The trick is getting the car parked on the mat each time, this method is easier for a motorcycle. Or a trashcan. 