Review `Scrollbars.kt`

[morozkin]

• [ ] Make sure the same approach for hoverDurationMillis is used among all scroll bars
• [ ] remember should be used with key as values are based on style and must reflect changes in style
• [ ] remember values instead of MutableState (val shape = remember { RoundedCornerShape(style.metrics.thumbCornerSize) } instead of val shape by remember { mutableStateOf(RoundedCornerShape(style.metrics.thumbCornerSize)) })

Discussed here