Dynamic BSN expressions generate a macro error

## Bevy version and features

bevy 0.19.0 with default features

## What you did

tried to reproduce the dynamic expression behavior from [the scene notation docs](https://docs.rs/bevy/latest/bevy/scene/index.html#using-dynamic-expressions-in-scenes)

```rs
use bevy::prelude::*;

fn main() {
App::new()
.add_plugins(DefaultPlugins)
.add_systems(Startup, setup)
.run();
}

#[derive(Component, FromTemplate)]
pub struct Health {
pub current: u32,
pub max: u32,
}

#[derive(Component, FromTemplate)]
pub struct Sprite {
pub image: Handle,
}

fn enemy(hp: u32, name: &str) -> impl Scene {
let name_string = name.to_string();
bsn! {
#{name}
Health { current: {hp / 2}, max: hp }
Sprite { image: {name_string + ".png"} }
}
}

fn setup(mut commands: Commands) {
commands.spawn_scene(bsn! { enemy(200, "goblin") });
}
```

## What went wrong

```
error: expected identifier
--> src/main.rs:24:10
|
24 | #{name}
| ^
error: could not compile `dynamic_expr_repro` (bin "dynamic_expr_repro") due to 1 previous error
```

i'm not sure if this is meant to be a currently supported feature or if it's not yet fully implemented and scheduled for a full release, but it may be helpful if the docs reflected that if so

Start with the dynamic expressions section of the scene notation docs and the minimal Rust reproduction shown in the issue. Compare the documented bsn! syntax with the macro's accepted input, then verify the result by rerunning the reproduction; done means the behavior is supported as documented or the documentation clearly states the limitation.

Written by the indexing model from the issue text.