Add merge_all_mesh_3d to world_asset_helpers

[IceSentry]

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

[IceSentry]

I added the contentious label because I expect some people will not like to put this in bevy_dev_tools