Add merge_all_mesh_3d to world_asset_helpers (#23710)

# Objective

- bevy_city needed a way to merge all the `Mesh3d` from a `WorldAsset`.
Instead of keeping that function hidden inside bevy_city we can make it
public

## Solution

- Add `world_asset_helpers` to `bevy_dev_tools`. It's in
`bevy_dev_tools` because it depends on multiple different parts of bevy
and that's the only crate that users can use that already depends on a
lot of things.
- Add `marge_all_mesh_3d()` to `world_asset_helpers`
- Update bevy_city to use that helper

## Testing

- I ran bevy_city and the meshes were merged like they should

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Co-authored-by: Carter Anderson <mcanders1@gmail.com>