feat(chart): Create sankey chart

[BrendanDavies]

SankeyChart Component

Adds a new SankeyChart component for visualizing flow data between nodes using ECharts.

Features

• Node & link visualization - Displays flow relationships with customizable node colors and link opacity
• Value labels - Optional display of values above node labels with customizable formatting
• Rich tooltips - Default tooltips with support for custom tooltipFormatter for full control
• Link color modes - gradient (blends source/target colors) or gray (flat gray links)
• Click handlers - onNodeClick and onLinkClick for interactive drill-down behavior
• Dark mode support - Automatic color adjustments via isDarkMode prop

Documentation

• Basic usage examples
• Multi-level flow demo
• Rich tooltip customization
• Interactive click handling
• Drill-down filtering pattern (click to filter by source node)

Dependencies

• Requires echarts as a peer dependency (not bundled)

[stritt]

@BrendanDavies can you resolve conflicts here so we can test?