Move internal-only time package under internal with compatibility plan

[shamaton]

github.com/shamaton/msgpack/v3/time is currently a public package, but it appears to be used only as an internal implementation detail for timestamp ext encoding/decoding.

Investigation results:

• GitHub code search found no external OSS imports of github.com/shamaton/msgpack/v3/time.
• pkg.go.dev reports no known importers for github.com/shamaton/msgpack/v3/time.
• v2/time also has no known importers on pkg.go.dev.
• Some older/forked repositories reference previous import paths, but they appear to be msgpack copies/forks rather than external consumers.

Proposed work:

• Move the current time package implementation under internal.
• Update internal imports to use the new internal path.
• For a minor release, consider leaving a thin deprecated compatibility shim at github.com/shamaton/msgpack/v3/time to avoid a technically breaking import-path removal.
• Document the change in the release notes, including that the package was not intended as public API.