Make power_preference_from_env() respect WGPU_POWER_PREF=none

[fornwall]

Also update wgpu/README.md now that default() is None.

Checklist

• Run cargo clippy.
• Run cargo clippy --target wasm32-unknown-unknown if applicable.
• Add change to CHANGELOG.md. See simple instructions inside file.

Description
Respecting WGPU_POWER_PREF=none makes no difference to wgpu testing and examples, as the fallback is already default() (which is PowerPreference::None), but makes a difference to other projects which for instance uses the logic "check for WGPU_POWER_PREF in the env, otherwise use high power".

Also updates wgpu/README.md to reflect that the default is now None.

Testing
Manually testing.

[teoxoy]

LGTM, thanks!