feat: add @? optional ingredient syntax

[hilli]

Summary

• Implements the @? operator for marking ingredients as optional, following the cooklang-rs extension convention
• Optional ingredients display with (optional) suffix across all output formats (HTML, Markdown, Print, default display)
• Includes comprehensive tests and example recipe demonstrating the feature

Changes

Token & Lexer (token/token.go, lexer/lexer.go)

• Added OPTIONAL_INGREDIENT = "@?" token type
• Lexer recognizes @? when followed by valid identifier, falls back to ILLEGAL otherwise

Parser (parser/parser.go)

• Added Optional bool field to Component struct
• Updated ingredient parsing to handle both @ and @? tokens

Core Types (cooklang.go)

• Added Optional bool field to Ingredient struct
• Updated Render() to output @? prefix for optional ingredients
• Updated RenderDisplay() to append (optional) suffix

Renderers

• HTML (renderers/html.go): Added optional CSS class and (optional) marker
• Markdown (renderers/markdown.go): Added *(optional)* suffix in ingredients list and inline
• Print (renderers/print.go): Added .optional and .optional-marker CSS, displays (optional)

Tests & Examples

• Added lexer tests for @? tokenization (6 test cases)
• Added parser tests for optional ingredients (6 test cases)
• Added spec tests in canonical_extensions.yaml (6 test cases)
• Created example_recipes/Gin_and_Tonic.cook demonstrating optional garnishes

Example Usage

Add @?thyme{2%sprigs} if desired.
Garnish with @?lime wedge{1} or @?lemon wheel{1}.

Output:

Ingredients:
  - 2 sprigs thyme (optional)
  - 1 lime wedge (optional)
  - 1 lemon wheel (optional)