Fix warning about nonzero_internals in vstd

[jaylorch]

This prevents getting warnings like the following:

"cargo" "verus" "verify" "--" "--verify-root"
warning: unexpected `cfg` condition value: `nonzero_internals`
  --> C:\Apps\verus\source\vstd\std_specs\mod.rs:39:7
   |
39 | #[cfg(feature = "nonzero_internals")]
   |       ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
   |
   = note: expected values for `feature` are: `alloc`, `allocator`, `allow_panic`, `default`, `std`, and `strict_provenance_atomic_ptr`
   = help: consider adding `nonzero_internals` as a feature in `Cargo.toml`
   = note: see <https://doc.rust-lang.org/nightly/rustc/check-cfg/cargo-specifics.html> for more information about checking conditional configuration
   = note: requested on the command line with `-W unexpected-cfgs`

warning: unexpected `cfg` condition value: `nonzero_internals`
   --> C:\Apps\verus\source\vstd\vstd.rs:152:11
    |
152 |     #[cfg(feature = "nonzero_internals")]
    |           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    |
    = note: expected values for `feature` are: `alloc`, `allocator`, `allow_panic`, `default`, `std`, and `strict_provenance_atomic_ptr`
    = help: consider adding `nonzero_internals` as a feature in `Cargo.toml`
    = note: see <https://doc.rust-lang.org/nightly/rustc/check-cfg/cargo-specifics.html> for more information about checking conditional configuration

warning: `vstd` (lib) generated 3 warnings (1 duplicate)
    Checking test4 v0.1.0 (C:\Apps\verus-systems-code\local\test4)
note: verifying root module

By submitting this pull request, I confirm that my contribution is made under the terms of the MIT license.

[Copilot]

Pull request overview

This PR updates the vstd crate’s Cargo feature definitions to match existing cfg(feature = "nonzero_internals") usage in the vstd sources, eliminating the Rust unexpected_cfgs warning about an unknown feature value when building vstd via Cargo.

Changes:

• Declares the nonzero_internals Cargo feature in source/vstd/Cargo.toml.

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[Chris-Hawblitzel]

Seems to be related to #2471