fix: restore registerDecls on Zig 0.16

[GrapeBaBa]

Motivation

registerDecls is publicly exported, but consumers cannot instantiate it on Zig 0.16. Its implementation declares a type return without returning a type, uses obsolete type-info tags, and still calls the pre-CallbackInfo(argc) function APIs. The string branch also exits module initialization before assigning the export.

Description

• Return void to match the registration side effect.
• Update type-info tags and callback construction for Zig 0.16 and the current createFunction API.
• Preserve the existing behavior of the options parameter.
• Register string declaration values before continuing through the remaining exports.
• Add a Node addon fixture and Vitest coverage for function and string declarations.