`rsx` macro should prevent any children in self-closing elements

## Feature Request

HTML includes several self-closing elements that are not designed to contain any child elements. Examples of such elements are:

```html

```

Currently, the `rsx!` macro in Dioxus allows content to be placed inside these self-closing elements:

```rust
pub fn component() -> Element {
rsx! {
img {"This is an image"}
}
}
```

## Implement Suggestion

The `rsx!` macro should produce an error if any content is nested inside self-closing elements. This would align the behavior of the macro with standard HTML practices and prevent potential rendering issues.