fix(chunk): update Zig AST node names to snake_case

[MattrHermanson]

What & why

Tree-sitter-zig parsers output AST node names in snake_case, but the plugin was checking for outdated PascalCase names. This PR updates the Zig node definitions so hlchunk.nvim can correctly identify and highlight code blocks (like functions, structs, and loops) in Zig files.

Changes

• Updated node definitions in lua/hlchunk/utils/ts_node_type/zig.lua to use snake_case naming.
• Added explicit node types for Zig structures such as function_declaration, struct_declaration, and enum_declaration to ensure accurate chunk boundary recognition.

Checklist

• make selene && make stylua && make test passes locally
• new behavior covered by test/features/*_spec.lua
• user-facing change reflected in docs/en and docs/zh_CN
• architectural change reflected in ARCHITECTURE.md / AGENTS.md