Remove indices from Mesh and into NewType

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

When creating a Mesh that have multiple materials, it is necessary to split that Mesh into multiple Meshes, each referencing a different material, that causes duplication of vertices.

## What solution would you like?

Create a new Asset `MeshIndices` and a new NewType `Mesh3dIndexed(Handle, Handle)`. `Mesh3d` would refer to meshes that does not have a indices list. The same for `Mesh2d`.

## Additional context

I tried looking into how Gltf stores the vertex buffers, if it is already split into the primitives or not, but the file format that I am using stores the vertices into a single buffer, and has a list of faces that references 3 vertices and the texture.