chore(gatsby): Fix Invalid 'main' field error (#31899)

(cherry picked from commit d06d6b5)

Author: sidharthachatterjee

packages/gatsby/src/internal-plugins/functions/index.ts renamed to packages/gatsby/src/internal-plugins/functions/index.js

@@ -2,7 +2,7 @@
   "name": "functions",
   "version": "1.0.0",
   "description": "An internal Gatsby plugin which lets one write serverless functions",
-  "main": "index.ts",
+  "main": "index.js",
   "scripts": {
     "test": "echo \"Error: no test specified\" && exit 1"
   },