feat: Implement Plugin Marketplace Manager and Client

- Added PluginMarketplaceManager to coordinate marketplace reads and manage installed plugins.
- Introduced PluginMarketplaceClient for fetching marketplace metadata.
- Implemented storage management for installed plugins using MemoryStorage.
- Added methods for installing, uninstalling, and enabling/disabling plugins.
- Created type definitions for PluginMarketplaceManager options and installed plugin records.
- Added support for fetching full marketplace catalog and individual plugin metadata.
- Included error handling for fetch operations and validation of plugin data.
- Updated index.browser.ts to export new modules and types.

dist/index.js

@@ -1,4 +1,6 @@
 export { MemoryStorage } from './interfaces.js';
+export { PluginMarketplaceClient } from './PluginMarketplaceClient.js';
+export { PluginMarketplaceManager } from './PluginMarketplaceManager.js';
 export { PluginTab } from './types.js';
 export { generateThemeCSS } from './theme-css.js';
 export { PluginRegistry } from './PluginRegistry.js';