docs: replace yarn recommendation

Author: dominikg

packages/playground/default-svelte-template/README.md

@@ -4,7 +4,7 @@ Created with `npx degit sveltejs/template`
 
 adapted to vite by moving index.html to root and replacing rollup config with vite
 
-use yarn
+use pnpm
 
-`yarn dev` starts dev server
-`yarn build` builds for production
+`pnpm dev` starts dev server
+`pnpm build` builds for production