fix(serve): check the 'Accept' header in the request before transform (fix #3502)

[oubenruing]

Description

this PR fix #3502

Additional context

For a project started in Vite's serve mode:
a. If there is a folder named "foo" with a package.json inside it in the root directory.
b. If there is a JavaScript file named "bar.js" in the root directory.
When accessing http://localhost:3000/foo or http://localhost:3000/bar, it will result in an error.

Here's a very simple example for any project started with Vite's serve mode, when accessing http://localhost:3000/vite.
Assuming my project has a route /vite, this will not give me the expected result.

This is because Vite does not check the source of the request during transformation. Considering the address entered in the browser's address bar, there no need for transformation; it should directly return the page itself. When a user directly enters an address in the browser, the Accept header will always include the "text/html" field. Therefore, in this pull request, we use the "Accept" header with "text/html" to make this determination.

What is the purpose of this pull request?

• Bug fix
• New Feature
• Documentation update
• Other

Before submitting the PR, please make sure you do the following

• 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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[stackblitz]

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[sapphi-red]

/ecosystem-ci run

[vite-ecosystem-ci]

📝 Ran ecosystem CI: Open

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

[oubenruing]

/ecosystem-ci run

It appears that some tests have not passed. What should I do to address this?

[sapphi-red]

qwik's fail is expected. But other fails seems to be caused by this PR.
We'll need to check why the tests are failing.
You can run the ecosystem-ci locally to take a deeper look: https://github.com/vitejs/vite-ecosystem-ci#via-shell-script

[sapphi-red]

marko fails with main branch so I guess that fail isn't related to this PR as well.

[oubenruing]

After using vite-plugin-html, this issue can be avoided, so close this PR.