[FEATURE] Add marker for top-level functions in Kotlin

[svenjacobs]

First of all thanks for this awesome plugin!

When developing Jetpack Compose, you work with a lot of (top-level) functions in a Kotlin file.

@Composable
fun MyScreen() {
  MyWidget()
}

@Composable
private fun MyWidget() {
}

It would be nice if functions (both public and private) can also be displayed as markers.