fix: add vercel.json

we need to have this to sepcify the build command so that the app can
build properly.

this should also be situated at the root dir, which is apps/web in this
case

Author: seaerchin

apps/web/vercel.json

@@ -0,0 +1,4 @@
+{
+  "$schema": "https://openapi.vercel.sh/vercel.json",
+  "buildCommand": "turbo vercel-build"
+}