fix: preserve TOML decorations/comments across set mutations - #12

- Import Array from toml_edit to enable array editing support
- Refactor set_value to route path mutations through array index handling or table-like handling
- Introduce helper handle_array_path to manage insert, replace, or remove for array items
- Introduce helper handle_tablelike_path to manage table-like mutations with inline table awareness
- Add insert_into_value_array to centralize insertion logic for ordinary arrays and handle value decoration
- Add insert_into_aot to handle insertion into arrays of tables with proper type checks
- Implement index bounds checks that return precise errors including the parent path
- Add remove_existing_array_item and remove_array_item_and_fix_format to cleanly remove items and fix formatting
- Add preserve_existing_value_decor to keep original decorations when replacing existing values
- Extend insert_tablelike logic to properly handle existing keys, inline tables, and associated decorations
- Add remove_inline_table_key_and_clean_comments to safely remove a key from an inline table and normalize remaining keys
- Refactor update_existing_tablelike_key and related code to manage inline table cleanup and key decoration consistently

Signed-off-by: rainbowatcher <rainbow-w@qq.com>