Config changes needed for stater template (#1680)

This does 2 things:

1. Adds prismjs as a dep.
2. Adds shiki as an external.

packages/astro/package.json

@@ -78,6 +78,7 @@
     "morphdom": "^2.6.1",
     "node-fetch": "^2.6.5",
     "path-to-regexp": "^6.2.0",
+    "prismjs": "^1.25.0",
     "remark-slug": "^7.0.0",
     "resolve": "^1.20.0",
     "sass": "^1.43.3",

packages/astro/src/core/create-vite.ts

@@ -18,6 +18,7 @@ const ALWAYS_EXTERNAL = new Set([
   'micromark-util-events-to-acorn',
   'node-fetch',
   'prismjs',
+  'shiki',
   'shorthash',
   'unified',
   'whatwg-url',