feat: support theming conversion funnel arrow via series.funnel.conve…

[erlanzhou]

🤔 This is a ...

• New feature
• Bug fix
• TypeScript definition update
• Enhancement feature
• Refactoring
• Other

🔗 Related issue link

N/A

🔗 Related PR link

N/A

💡 Background and solution

The conversion funnel (conversionFunnel, @visactor/vchart-extension) draws each conversion-rate connector as three marks — leader line (polygon), arrow head (symbol), label (text). Their styles were hardcoded and only overridable via spec.conversionArrow.*; there was no theme-level entry point.

• Add theme token series.funnel.conversionArrow (line / symbol / text) to IFunnelSeriesTheme, with built-in defaults in the common funnel theme.
• Defaults equal the previous hardcoded constants (line.stroke: black, symbol.fill: black, text: #606773 / 12) → non-breaking; existing charts render unchanged.
• The extension transformer reads getTheme('series','funnel').conversionArrow and injects it between defaults and spec. Merge order: default → theme → spec (spec wins).
• Themeable: style attributes only. Runtime-computed attributes stay locked (line points, symbol x/y/angle, text auto-position).

new VChart(spec, {
  theme: {
    series: {
      funnel: {
        conversionArrow: {
          line:   { style: { stroke: '#00AA55', lineWidth: 2 } },
          symbol: { style: { fill: '#E8384F', symbolType: 'arrow', size: 12 } },
          text:   { style: { fill: '#2E5CFF', fontSize: 14 } }
        }
      }
    }
  }
});

📝 Changelog

Language	Changelog
🇺🇸 English	feat: support theming the conversion funnel conversionArrow (line / symbol / text) via the series.funnel.conversionArrow theme token (defaults unchanged, non-breaking)
🇨🇳 Chinese	feat: 支持通过 series.funnel.conversionArrow 主题 token 配置转化漏斗转化率箭头（引导线 / 箭头 / 文字）样式（默认值不变，向后兼容）

☑️ Self-Check before Merge

• Doc is updated/provided or not needed
• Demo is updated/provided or not needed
• TypeScript definition is updated/provided or not needed
• Changelog is provided or not needed