respect `WGPU_ADAPTER_NAME` environment variable

[ickk]

wgpu has a way to filter the underlying GPU using an environment variable https://docs.rs/wgpu/latest/wgpu/util/fn.initialize_adapter_from_env_or_default.html

This is an incredibly useful feature for testing an application's performance with different GPUs, and for checking for compatibility and bugs that might be present on different system configurations.

Bevy could allow use of this environment variable to aid development & testing

[ickk]

I thought it might be possible for an application to programmatically configure the adapter with WgpuSettings.device_label, however after looking into it I think that field is just for creating a debug label for the device and does not influence which device is chosen

[ickk]

also note that the PowerPreference enum is not sufficient. There exists several physical devices that I want to select between, but this setting only allows flipping between "high-performance" & "low-power"