fix(resolve)!: remove special .mjs handling

[bluwy]

Description

fix #14404

When an "exports" field maps to a .mjs file, continue using that file and don't try to resolve the "browser" or "module" fields.

Additional context

---

What is the purpose of this pull request?

• Bug fix
• New Feature
• Documentation update
• Other

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• Read the Contributing Guidelines.
• Read the Pull Request Guidelines and follow the PR Title Convention.
• Check that there isn't already a PR that solves the problem the same way to avoid creating a duplicate.
• Provide a description in this PR that addresses what the PR is solving, or reference the issue that it solves (e.g. fixes #123).
• Ideally, include relevant tests that fail without this PR but pass with it.

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[bluwy]

/ecosystem-ci run

[vite-ecosystem-ci]

📝 Ran ecosystem CI on 3155e3a: Open

suite	result	latest scheduled
analogjs	✅ success	✅ success
astro	✅ success	✅ success
histoire	✅ success	✅ success
ladle	✅ success	✅ success
laravel	❌ failure	❌ failure
marko	❌ failure	❌ failure
nuxt	✅ success	✅ success
nx	❌ failure	❌ failure
previewjs	✅ success	✅ success
qwik	✅ success	✅ success
rakkas	✅ success	✅ success
sveltekit	❌ failure	❌ failure
unocss	✅ success	✅ success
vike	✅ success	❌ failure
vite-plugin-pwa	❌ failure	❌ failure
vite-plugin-react	✅ success	✅ success
vite-plugin-react-pages	✅ success	✅ success
vite-plugin-react-swc	✅ success	✅ success
vite-plugin-svelte	✅ success	✅ success
vite-plugin-vue	✅ success	✅ success
vite-setup-catalogue	✅ success	✅ success
vitepress	✅ success	✅ success
vitest	✅ success	❌ failure

[ArnaudBarre]

I can't say I understand all the consequences, but less code to better match the spec, I'm all in