fix: custom event payload type

[patak-dev]

Description

Fix VitePress.

Looks like https://github.com/vitejs/vite/pull/7476/files#diff-5bc77abd0513da66cf1ce3d8c50e8108daf0c149a5f5f973eeea3c9d4a47bffbR16 broke VitePress.

See vite-ecosystem-ci run https://github.com/vitejs/vite-ecosystem-ci/runs/5714794922?check_suite_focus=true#step:6:384.

Better message when running locally:

> vitepress@0.22.3 build-client /Users/patak/vitepress
> tsc -p src/client && node scripts/copyClient

src/client/app/index.ts:134:29 - error TS2571: Object is of type 'unknown'.

134         router.route.data = payload.pageData
                                ~~~~~~~
Found 1 error.

Additional context

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What is the purpose of this pull request?

• Bug fix
• New Feature
• Documentation update
• Other