What to specify for mainClass when there is none?

### Motivation
It would be helpful if the tooltip showed what to do when there is no main class.

### Description
I created a new directory, opened it, and created "main.kt"

```
fun main(args: Array) {
println("hello?");
}
```

Then, I selected the default "kotlin" launch script, and it had "mainClass" setting. But my file has no main class. Tried ` "mainClass": "${workspaceFolder}\\main.kt"`, but it did not work.

### Alternatives considered
Creating a class. But that's cumbersome.

Start with the default "kotlin" launch script and its `mainClass` setting, using the reported `main.kt` example as the reproduction case. Trace how the tooltip presents this setting and determine what guidance is needed when the entry point is a top-level Kotlin `main` function. Done means the tooltip explains the valid configuration and the launch setup works for this example.

Written by the indexing model from the issue text.