feat(virtual-scroll): implement virtual scrolling for diagnostics list

[Muromi-Rikka]

• Replace Checker component with Diagnostic in virtualized list
• Add @tanstack/vue-virtual dependency for virtualization logic
• Style adjustments for .message-item and .message elements
• Update package.json to include new dependency

https://github.com/fi3ework/vite-plugin-checker/issues/543

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