fix(astro): Mark SDK package as Astro-external (#9509)

Astro's Vite plugin tries to detect if a package is an "astro component
package" (i.e. contains `.astro`) files by checking for a bunch of
heuristics in the project's `package.json`. These packages will be run
through the Astro compiler.


https://github.com/withastro/astro/blob/7c458514c06c95158245bba4fa3c254abd333f5a/packages/astro/src/core/create-vite.ts#L74-L89

Because our SDK package matches multiple of the used heuristics, it is
added to the compiler build which causes errors because some of our
packages make the build fail (not sure why/how). Apparently, this is
only problematic when the project is managed by `pnpm` which results in
a build error. Other package managers don't seem to have a problem.

packages/astro/package.json

@@ -74,5 +74,8 @@
   },
   "volta": {
     "extends": "../../package.json"
+  },
+  "astro": {
+    "external": true
   }
 }