Allow the creation of handles from runtime values in bsn!

## What problem does this solve or what need does it fill?

You will still need to pass a handle from outside or add it manually in `template(...)`.

```rust
pub fn demo(atlas: Handle) -> impl Scene {
bsn! {
ImageNode {
image: "images/button.png",
texture_atlas: { TextureAtlasTemplate {
layout: HandleTemplate::Handle(atlas.clone()),
index: 0,
}},
}
}
}

// or
pub fn demo2() -> impl Scene {
bsn! {
template(|ctx| {
Ok(ImageNode {
image: ctx.resource::().load("images/button.png"),
texture_atlas: Some(TextureAtlas {
layout: ctx.resource_mut::>().add(
TextureAtlasLayout::from_grid(UVec2::new(65, 12), 1, 3, None, None)
),
index: 0,
}),
..default()
})
})
}
}
```

Allowing create a handle in-place when authoring a scene function. It is more ergonomic to compose related data of one component.

## What solution would you like?

Add a `HandleTemplate::Insert(...)` to create a handle directly from value.
```rust
pub fn demo() -> impl Scene {
bsn! {
ImageNode {
image: "images/button.png",
texture_atlas: { TextureAtlasTemplate {
layout: HandleTemplate::Insert(TextureAtlasLayout::from_grid(UVec2::new(65, 12), 1, 3, None, None)),
index: 0,
}},
}
}
}
```

## Additional context

This may also works when composing a bsn file, I think.

Start by reading the bsn! and template handling around HandleTemplate, then inspect how runtime values are currently converted into handles. Compare the existing HandleTemplate::Handle path with the proposed HandleTemplate::Insert behavior and check the examples in the issue. Done means a handle can be created in place from a runtime value while preserving the existing external-handle path.

Written by the indexing model from the issue text.