feat: add spans support to Text widget for inline rich text and widget embedding

[TheNoumanDev]

Summary

• Extends the existing Text widget with a new spans property that enables rich text rendering with mixed styling and inline widgets (e.g., Icons, Images) within a single text flow
• When spans is provided, the widget internally uses Text.rich() with TextSpan and WidgetSpan children; existing text: usage remains fully unchanged
• Adds span_definition.dart as a lightweight data class for parsing span entries from raw YAML

YAML Syntax

Text:
  spans:
    - text: "Read our "
    - text: "Terms of Service"
      textStyle:
        color: 0xff4b93e7
        decoration: underline
      onTap:
        navigateScreen:
          name: HelpScreen
    - widget:
        Icon:
          name: info
          size: 18
  styles:
    textStyle:
      fontSize: 16

Span Types

• Text spans — text, optional textStyle override, optional onTap action
• Widget spans — any Ensemble widget (Icon, Image, etc.) rendered inline via WidgetSpan