feat: support multi line strings

[archiewood]

This adds Python-style triple-quoted strings ("""...""") for multi-line attribute values. Useful for SQL queries and other content that contains quotes or spans multiple lines.

Example:

{% table 
  data="demo_daily_orders"
  series="""
      case
          when sales > 18000 then 'High'
          when sales > 9000 then 'Medium'
          else 'Low'
      end
  """ 
/%}

Changes:

• Updated PEG grammar to support """ delimiters
• Enhanced tokenizer to handle triple-quote boundaries
• Added tests for multi-line strings, embedded quotes, and interpolation