fix: opt out of direct import in SSR when the file is not JS

[cyco130]

Description

Currently importing external CSS files in SSR throws ERR_UNKNOWN_FILE_EXTENSION unless those modules are marked as external (reproduction, StackBlitz). This PR solves it by opting out of direct Node import if the resolved file has a non-JS extension.

Additional context

The regex testing if a file is a non-JS file (which I copied from the optimizer code) may require some more thinking.

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What is the purpose of this pull request?

• [x] Bug fix
• [ ] New Feature
• [ ] Documentation update
• [ ] Other

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

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

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[bluwy]

I tested the repro with Vite 2.9.13 and plugin-react 1.3.2, and it seems to work fine. Stackblitz. So this might be a regression in Vite 3.