build: add pnpm as packageManager

Author: tmlmt

package.json

@@ -3,6 +3,7 @@
   "author": "Thomas Lamant <[email protected]> (https://github.com/tmlmt)",
   "description": "Thomas Lamant's homepage",
   "repository": "github:tmlmt/homepage",
+  "packageManager": "[email protected]",
   "private": false,
   "scripts": {
     "build": "nuxt build",