[WIP] Fix issue with Vite plugin React rendering

[Copilot]

Implement dynamic switching to user-installed react-server-dom-webpack (Issue #913)

Progress

• Create a utility function to check if react-server-dom-webpack is installed in user's project
• Add resolve logic to dynamically choose between user's package and vendored version
• Update import paths in source files to use the dynamic resolution
• Update plugin configuration to handle both scenarios
• Update tsdown.config.ts to still vendor the package as fallback
• Test builds work with vendored version (backward compatibility)
• Test with user-installed react-server-dom-webpack
• Document the new behavior

Implementation Details

The solution uses a Vite plugin-based approach:

1. Source files import from @vitejs/plugin-rsc/vendor/react-server-dom/*
2. The rsc:resolve-react-server-dom plugin intercepts these imports
3. If user has react-server-dom-webpack installed, it redirects to react-server-dom-webpack/*
4. Otherwise, it uses the vendored version as fallback
5. The getReactServerDomImportPath() helper checks user's project for the package

This approach:

• Maintains backward compatibility
• Only runs resolution logic in Node.js (plugin context)
• Doesn't affect browser builds
• Uses standard Vite resolution mechanisms

Original prompt

implement @vitejs/vite-plugin-react/issues/913

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[hi-ogawa]

Closing as #915 seems better