fix: force module format for virtual client-proxy file (#74162)
Fixes #74062 (`jotai` ran into this error [when they added `"type":
"commonjs"` to their
package.json](https://github.com/pmndrs/jotai/discussions/2579#discussioncomment-9812492))
> this is a bug in the transform we do when a `'use client'` directive
is encountered. I think what's happening is that we're creating a
virtual file that uses ESM import/export syntax, but it's called
proxy.js (not proxy.mjs), so the `"type": "commonjs" `makes turbopack
"correctly" upset at the ESM syntax.
https://github.com/vercel/next.js/issues/74062#issuecomment-2555922867
The (slightly kludgy) solution is to use `proxy.mjs` or `proxy.cjs` to
force the module format, bypassing the issue where `proxy.js` changes
meaning depending on `package.json#type`.
