feat: add --write-meta flag to copy meta.nix files

[vic]

Add a new --write-meta CLI option that enables not removing flake-parts/<part>/meta.nix files. This is useful for other Nix tools that can take advantage of these meta files.

This option comes handy for projects using github:vic/flake-file that maintain flake.nix on behalf of the user. Because of this, modifying the flake.nix file is undesirable, however, having the meta files available (example) allows seamless integration with flake-parts-builder, letting users take advantage of such templates, while also automatically managing inputs read from meta files.

• Add write_meta field to SharedArgs struct with documentation
• Thread write_meta parameter through copy_parts_to_target function
• Implement meta.nix file copying logic with proper error handling
• Support existing init strategies (skip/overwrite) for meta files
• Update both init and add commands to use the new functionality

Testing

Tested manually by invoking init and add commands with the new option.

Dependencies

None

Screenshots

None

Checklist

• I have tested the relevant changes locally.
• I have checked that nix flake check passes.
• I have ensured my commits follow the project's commits guidelines.
• I have checked that the changes follow a linear history.
• (If applicable) I have commented any relevant parts of my code.
• (If applicable) I have added appropriate unit/feature tests.
• (If applicable) I have updated the documentation accordingly (in English).

Additional Notes

None