feat: add ThemeEngine tests and implement ThemeContext for theme management

- Created comprehensive tests for ThemeEngine functions including color conversion, typography, border radius, shadows, animations, z-index, theme generation, merging, and inheritance.
- Introduced ThemeProvider and context for managing themes and modes in React, supporting theme switching, persistence, and CSS variable injection.
- Implemented hooks for accessing theme context and optional theme usage.
- Added tests for ThemeProvider to ensure proper functionality and persistence behavior.

Author: hotlong

packages/core/src/index.ts

@@ -17,6 +17,7 @@ export * from './evaluator/index.js';
 export * from './actions/index.js';
 export * from './query/index.js';
 export * from './adapters/index.js';
+export * from './theme/index.js';
 // export * from './data-scope'; // TODO
 // export * from './validators'; // TODO