fix: should load --config foo.mjs as an ES module

[sodatea]

Description

This PR fixes the following bug:
When a user names their config file as vite.config.mjs, running vite directly can correctly load the file as an ES module, but vite --config vite.config.mjs would try to transpile that file to CJS first.

This bug was first mentioned at #4455 (comment)

Additional context

I didn't find any existing tests for config file loading, so I didn't add one for this issue either.

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What is the purpose of this pull request?

• Bug fix
• New Feature
• Documentation update
• Other

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• Read the Contributing Guidelines.
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• Check that there isn't already a PR that solves the problem the same way to avoid creating a duplicate.
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