backport: force module format for virtual client-proxy (#74162)

[lubieowoce]

Fixes #74062 (jotai ran into this error when they added "type": "commonjs" to their
package.json)

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.
#74062 (comment)

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.

[ijjk]

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	vercel/next.js 14-2-1	vercel/next.js lubieowoce/backport-fix-client-transform-mjs	Change
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build cache

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0.pack gzip	1.63 MB	1.63 MB	N/A
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Overall change	0 B	0 B	✓

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Commit: 0aa8020

[eps1lon]

rust check ain't happy

[ijjk]

Tests Passed