Clean up with-supabase example (#61799)
### What?
[1] Clean up `with-supabase` example following patterns from @leerob's
recent video on authentication
[2] Add images to matcher to reduce number of times a session is
refreshed
[3] Move instantiation of `cookieStore` into Supabase server helper
### Why?
[1] Make template easier to understand and use
[2] Reduce likelihood users will receive Auth Rate Limit Exceeded error
[3] Makes creating a Supabase client much simpler in Server Components,
Route Handlers and Server Actions
### How?
[1] Refactor middleware and its helper function
[2] Add image extensions to middleware matcher
[3] Call `cookies()` function from `createClient` helper - this should
still be in the [same execution
context](https://nextjs.org/docs/messages/dynamic-server-error) and
appears to work from my testing, but would be good to get the 👍 from
someone at Vercel
