Add shopping list parser and serializer

[dubadub]

Summary

• Add new shopping_list module behind a shopping_list feature flag (enabled by default)
• Parse and serialize a tree-structured shopping list format with recipe references (./path{multiplier}) and free-hand ingredients (name{qty%unit})
• Support nested items via 2-space indentation, comments (//), bare ingredients, and fractional multipliers

Format

./Breakfast/Easy Pancakes{2}
  ./Some/Nested Recipe{2}
free hand ingredient{4%l}
salt

API

• shopping_list::parse(input) -> Result<ShoppingList, ShoppingListError> — strict parser
• shopping_list::write(list, writer) -> io::Result<()> — serializer (round-trip tested)
• ShoppingListError implements RichError for annotated error reporting
• All types implement Serialize/Deserialize

Test plan

• 19 unit tests covering basic parsing, nesting, round-trip serialization, and error cases
• Full test suite passes (cargo test --all-features, excluding pre-existing serde_test failure)
• No new clippy warnings

[pkg-pr-new]

• cooklang-playground

  npm i https://pkg.pr.new/cooklang/cooklang-rs/@cooklang/cooklang@89
  

commit: ed8f171