Use LazyLock for std environments and OnceBox for no_std

[FalkWoldmann]

Description
Right now, we always use once_cell in naga, even when std with LazyLock is available.

Testing
I ran cargo xtask test locally.

Squash or Rebase?

Rebase

Checklist

• Run cargo fmt.
• Run taplo format.
• Run cargo clippy --tests. If applicable, add:
  • --target wasm32-unknown-unknown
• Run cargo xtask test to run tests.
• If this contains user-facing changes, add a CHANGELOG.md entry.

[FalkWoldmann]

The "cleanest" way of ensuring that once_lock is actually optional for std builds would be to introduce an explicit std feature. This would, however, mean breaking changes for no_std users, as they would then need to add naga with no default features and the once_cell feature. Would this be okay with you?

[ErichDonGubler]

@FalkWoldmann: Yes, please! Filed #8701 for tracking. 🙂

[ErichDonGubler]

This code overall LGTM, but before I merge: What's the motivation for making this change? Is it to eliminate an unused dependency?