feat: Add VxTextTheme for theme-level default text styling (#7)

[divyanshbhai]

Fix #7

Summary

Implements VxTextTheme to allow setting default text styles at theme level for VxTextBuilder widgets.

Changes

• ✅ Added VxTextTheme InheritedWidget class
• ✅ Integrated VxTextTheme with VxTextBuilder
• ✅ Added comprehensive tests
• ✅ Updated example app with usage demo
• ✅ Added documentation

Usage

VxTextTheme(
  defaultTextStyle: TextStyle(
    fontSize: 16,
    color: Colors.blue,
    fontWeight: FontWeight.w500,
  ),
  child: MaterialApp(home: MyApp()),
)


Fix #7