feat: honor the omitzero struct-tag option

[gdamore]

Summary

encoding/json gained the omitzero field option in Go 1.24: a field is omitted when it holds the zero value for its type, as determined by an IsZero() bool method if present, otherwise by reflect.Value.IsZero(). msgpack's tag parser already recognizes the option (tag.HasOption("omitzero")), but it was silently ignored, so a zero-valued field tagged ,omitzero was always encoded.

This honors omitzero in field.Omit, independently of omitempty.

Details

• A new isZeroValue helper mirrors encoding/json's omitzero semantics:
  • a type's own IsZero() takes precedence;
  • a nil pointer/interface is treated as zero up front, so a typed-nil whose type carries an IsZero method (e.g. (*time.Time)(nil)) is not dereferenced — the same guard encoding/json uses;
  • otherwise reflect.Value.IsZero() decides.
• Unlike omitempty's isEmptyValue, it does not treat a non-nil empty slice, map, or string as omittable — omitempty and omitzero are intentionally distinct, as in encoding/json.
• Works with a custom struct tag (e.g. SetCustomStructTag("json")) too, since the option is parsed generically.

Test

TestOmitZero covers int/string/bool zero values, a type with a value-receiver IsZero, and a nil *time.Time (exercising the nil-pointer guard). It fails on v5 without this change and passes with it.

Notes

Related to #390, which discusses aligning omit semantics with encoding/json and notes omitzero as the path Go took. This PR adds the omitzero option specifically; it does not change existing omitempty behavior.