v0.0.1 of shadcn_theme

Theme-only Flutter package for shadcn color themes and style presets. Changelog excerpt: - Replaced the template package API with a theme-only shadcn Flutter API. - Added 24 official shadcn color themes with light and dark semantic tokens. - Added 7 official shadcn style presets covering sizing, spacing, radius, typography, borders, shadows, and state layers. - Added ShadcnThemeData.lightand ShadcnThemeData.darkfactories. - Added ShadcnThemeExtensionfor reading active tokens from ThemeData. - Added unit tests for registry completeness, representative token values, `ThemeData[...]