Astro integration

Since Astro supports [middleware], we could integrate Fedify with Astro as well.  For easier integration, it would be great if Fedify provides an integration function like `@fedify/fedify/x/astro`.

Some context: <https://github.com/dahlia/fedify/discussions/37#discussioncomment-9372276>.

[middleware]: https://docs.astro.build/en/guides/middleware/#the-context-object