chore: watch for messages changes in dev generate script (#15950)

* chore: watch for messages changes in dev generate script

* no need for this to be async

* tweak

* guard

* only create one timeout

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Co-authored-by: Rich Harris <rich.harris@vercel.com>

Author: paoloricciuti

packages/svelte/package.json

@@ -136,7 +136,7 @@
   ],
   "scripts": {
     "build": "node scripts/process-messages && rollup -c && pnpm generate:types && node scripts/check-treeshakeability.js",
-    "dev": "node scripts/process-messages && rollup -cw",
+    "dev": "node scripts/process-messages -w & rollup -cw",
     "check": "tsc --project tsconfig.runtime.json && tsc && cd ./tests/types && tsc",
     "check:watch": "tsc --watch",
     "generate:version": "node ./scripts/generate-version.js",