Warn if user declares a CssStyle / Keyframes instance anywhere but the top level

[bitspittle]

For example

@Page
@Composable
fun SomePage() {
   val ExampleStyle = CssStyle { ... }
}

Styles must be declared at the top level (or inside an object), but we can't detect and warn about this case because KSP doesn't give us insight into what is happening inside methods. So we can use the IDE plugin to capture these cases.

We might also provide an action to move these properties up to the top level.