[frameworks] Add default immutable cache path for Astro v2 (#9305)

Astro v2 was released today. It includes [improved support for caching
all hashed build
assets](https://docs.astro.build/en/guides/upgrade-to/v2/#changed-_astro-folder-for-build-assets)
by gathering these all in a single `_astro` directory in build output
(previously these ended up in a number of different places).

This PR updates the Vercel frameworks config to provide out-of-the-box
immutable caching for these assets.

Co-authored-by: Steven <steven@ceriously.com>

packages/frameworks/src/frameworks.ts

@@ -272,6 +272,11 @@ export const frameworks = [
         headers: { 'cache-control': 'public, max-age=31536000, immutable' },
         continue: true,
       },
+      {
+        src: '^/_astro/(.*)$',
+        headers: { 'cache-control': 'public, max-age=31536000, immutable' },
+        continue: true,
+      },
     ],
   },
   {