You use the wrong path to STLLoader. The module version of the loader is in the jsm directory (not js).
import { STLLoader } from 'three/examples/jsm/loaders/STLLoader.js'
You use the wrong path to STLLoader. The module version of the loader is in the jsm directory (not js).
import { STLLoader } from 'three/examples/jsm/loaders/STLLoader.js'