Consider moving from negative `no_std` feature to positive `std` feature

[ErichDonGubler]

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?

Originally posted by @FalkWoldmann in #8630 (comment)

---

@FalkWoldmann: Yes, that would be okay! 😀

In general, it's best to follow the official Cargo guidelines for features; in Feature unification, they say:

A consequence of this is that features should be additive. That is, enabling a feature should not disable functionality, and it should usually be safe to enable any combination of features. A feature should not introduce a SemVer-incompatible change.