Avoid leaking Vite plugin types

[peterp]

Problem

Some projects can end up with two physical copies of Vite in node_modules. When that happens, TypeScript may see RedwoodSDK's Vite plugin type and the app's Vite plugin type as two different things. The app then gets a confusing defineConfig() type error, even though the plugins work at runtime.

Reported by @quantizor in #1112.

Solution

This PR follows the dependency-layout fix suggested in #1112: make @vitejs/plugin-react an optional peer dependency instead of a runtime dependency of rwsdk. That lets the app own the React Vite plugin and its Vite peer, avoiding an extra Vite copy under rwsdk/node_modules.

Technical Summary

• Move @vitejs/plugin-react from dependencies to optional peerDependencies and devDependencies in sdk/package.json.
• Lazy-load @vitejs/plugin-react only when RedwoodSDK needs to include the default React Vite plugin.
• Keep the typed redwoodPlugin() return type as Promise<InlineConfig["plugins"]>, addressing the review concern about any[].
• Add @vitejs/plugin-react to the starter app so generated apps satisfy the optional peer.
• Default to including RedwoodSDK's React plugin unless users explicitly pass includeReactPlugin: false.

Closes #1112

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[justinvdm]

@peterp Can't we import type InlineConfig and then use it here? any casting for our main redwoodPlugin export is quite a change.

[peterp]

Yeah! I don't know why the inference picked this shape.