#3530837: Mark React external in Astro's build, don't use @astrojs/preact's `compat` option

I wanted to explore why we needed to use the compat option with the @astrojs/preact plugin in the first place, because it is what's causing Astro's bundler to rename the exports of preact/compat, supposedly to avoid name collisions.

By marking React external in the Astro config we can rely on the same import maps as the code components do, and that will also ensure the compatibility for any code we bundle.