fix(next-plugin): Next.js 13.5.x build not outputting css on windows

[syfxlin]

CSS output is not working again in Next.js 13.5.x and Windows.
The resolvedNextConfig.experimental.appDir is no longer available in Next.js 13.5.x, so this pre-condition has been removed.

References: https://github.com/vercel/next.js/blob/76e06d55a8c42090dafda25ec31d9eeaa7ec647c/packages/next/src/build/index.ts#L418-L422

[changeset-bot]

🦋 Changeset detected

Latest commit: 602b1d0

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

Name	Type
@vanilla-extract/next-plugin	Patch

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[ha1fstack]

I think comments can be changed a bit

// https://github.com/vercel/next.js/blob/5b52e7772dd6237474931ba4bbb856fbc33142ad/packages/next/src/build/index.ts#L431
// https://github.com/vercel/next.js/blob/5b52e7772dd6237474931ba4bbb856fbc33142ad/packages/next/src/build/webpack-config.ts#L493
// https://github.com/vercel/next.js/pull/43916
// on Next.js 12, findPagesDirResult is a string. on Next.js 13, findPagesDirResult is an object
const findPagesDirResult = findPagesDir(
  dir,
  resolvedNextConfig.experimental?.appDir ?? false,
);
// Skip nextConfig check since appDir is stable feature after Next.js 13.4
const hasAppDir = !!(findPagesDirResult && findPagesDirResult.appDir);

[askoufis]

LGTM. Feel free to add the comments suggested in #1193 (comment), but they shouldn't block the PR.

[syfxlin]

Hi @askoufis, I added the comment suggested in add #1193, but I don't have access to merge pull request, can you help with that?