Can no longer construct a `naga::proc::Namer`

#7338, released with naga 25.0.0, changed the `Default` impl of `naga::proc::Namer` to be conditional on

```rust
#[cfg(any(wgsl_out, glsl_out, msl_out, hlsl_out, test))]
```

This means that an out-of-tree backend cannot construct a `Namer` without spuriously enabling one of those features. I assume this is an unintended consequence.