feat: Add nested data support for line charts

[skokenes]

Summary

This PR adds support for nested data structures in line charts, allowing visualization of hierarchical Malloy query results. Users can now specify nested field paths using intuitive dot notation (e.g., x='monthly_sales.dcId').

Key Changes

• Dot notation support: Users can specify nested fields using field1.field2 syntax in viz tags
• Automatic field detection: Intelligently detects series, x, and y fields from nested data structures
• Data flattening: Transforms nested Malloy data into a flat structure suitable for Vega visualization
• Placeholder Field objects: Creates proper Field interface implementations for nested fields with all required properties including tag

Example Usage

# viz=line { series=department x='monthly_sales.dcId' y='monthly_sales.total_sales' }
view: nested_line_explicit_paths is {
  group_by: department
  nest: monthly_sales is {
    group_by: dcId
    aggregate: total_sales
  }
}

Implementation Details

• Field paths are converted from dot notation to JSON arrays internally for compatibility with Malloy's path resolution
• Nested data is flattened in the mapMalloyDataToChartData function before passing to Vega
• Auto-detection prioritizes nested fields when available, using outer dimensions for series and inner fields for x/y axes
• Fields tagged with "tooltip" are excluded from automatic detection

Testing

• Updated existing story examples to use dot notation
• All builds pass successfully
• Line charts render correctly with nested data

[lloydtabb]

Love this. Going to be a big deal when it is done.