allow symlinked assets

[gaarf]

This might fix #4709

my monorepo setup uses symlinked static assets, and it worked great until #5047 !
I believe that check for the casing was too restrictive, so I went ahead and made this PR addressing the issue. There's a test included in the basics app. The issue really only applies to dev, since the build copies symlink contents transparently.

Thanks in advance for the review 🙏

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[gaarf]

@anarchodin @mrkishi @Rich-Harris please have a look 🪟 🔗 ⬆️

[Rich-Harris]

thank you! the correct fix looks a bit different — we can't just check the end of the path, because that won't work if the symlink source and target have different names. instead we're just checking that file exists in fs.readdirSync(path.dirname(file)), which is only true if the case is correct (and then doing that recursively)