feat(css): warn if url rewrite has no importer

[bluwy]

Description

Warn if there's no importer when rewriting URLs as it causes issues like in #7186

Additional context

Example:

vite v3.0.0-alpha.0 building for production...
transforming (1) index.html
A PostCSS plugin did not pass the `from` option to `postcss.parse`. This may cause imported assets to be incorrectly transformed. If you've recently added a PostCSS plugin that raised this warning, please contact the package author to fix the issue.
✓ 11 modules transformed.
dist/index.html                  0.42 KiB
dist/assets/index.c5ab1dfe.css   0.22 KiB / gzip: 0.19 KiB
dist/assets/index.0f88313c.js    50.34 KiB / gzip: 20.31 KiB

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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 Commit 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.

[patak-dev]

I'm a little bit worried about how many plugins in the postCSS ecosystem have this issue. But I think we could merge it to get some reports from users during the beta. And we can decide before releasing v3 if we should remove this log.

[bluwy]

Without the importer, it is breaking the build output though so users would see runtime errors when previewing. So it may be better to report it beforehand as it's something we know would happen. Don't think many modern plugins would hit this though.