fix: astro check fails on @vercel/speed-insights/astro missing types

[feugy]

📓 What's in there?

Fixes #36. It is not possible yet to publish types along an Astro component, and the astro check command will fail on it. As per the Astro team recommendations, we need to specify a file for types, even if it does not exist.

🧪 How to test?

1. once the code got retrieved locally, build the whole package: pnpm -F @vercel/speed-insights build
2. then go to the astro example app cd apps/astro
3. install typescript and the check command: pnpm i -D typescript @astrojs/check

4. run it: pnpm exec astro check: it will work, while it used to fail with error:

   src/components/BaseHead.astro:5:27 - error ts(2307): Cannot find module '@vercel/speed-insights/astro' or its corresponding type declarations.
   
   5 import SpeedInsights from '@vercel/speed-insights/astro';

Note: there is still a warning about speedInsightsBeforeSend() unused function, but I couldn't get rid of it.

[vercel[bot]]

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