Add codemod for `middleware` to `proxy` (#84127)
Stacked to https://github.com/vercel/next.js/pull/84119
This PR adds a codemod for renaming `middleware` to `proxy`. Below are
the behaviors:
- Rename `middleware.*`, `src/middleware.*` to `proxy.*`, `src/proxy.*`
- Rename function/variable exports from `middleware` to `proxy`. Default
exports are skipped.
