In previous versions of Android Studio was it possible to run an independent main-function by typing fun main() somewhere in a file. Android Studio then showed a run-button on the left vertical bar and you could run the code-snippet within main() { ... } separate from the actual project.

Currently I have updated to Android Studio Quail 4. The feature seems to be gone. At least the run-button doesn't appear.

Was the feature removed or how has it to used now?

fun main() {
    println("Testing, 123 ...")
}