fix(css): return sourcemap in vite:css transform

[dominikg]

Description

return actual sourcemap for css transform instead of a dummy

Additional context

may fix #649

Note i didn't do any further analysis besides running the tests, but this seems like a trivial change for the better, can't get more broken than that dummy.

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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.

[dominikg]

There was a previous attempt at this aimed at vite1 that was closed #659 and there may be other things to consider that i missed.
Thanks to @bluwy who reminded me about this.

[patak-dev]

I wonder how this wasn't caught before by someone

[hex-ci]

This PR seems to have an impact on the vite-plugin-vue2 plugin. If you upgrade to Vite 2.5.7, a "Sourcemap is likely to be incorrect" warning will be reported during build. If you downgrade to Vite 2.5.6, there will be no warning, only downgrade vite-plugin-vue2 There are always warnings. I haven't figured out the specific reason, and I need help, thank you.

[patak-dev]

@hex-ci did you open an issue in vite-plugin-vue2? Maybe this is something to be solved there? Please link the issue for reference here

[hex-ci]

@hex-ci did you open an issue in vite-plugin-vue2? Maybe this is something to be solved there? Please link the issue for reference here

I created an issue: underfin/vite-plugin-vue2#132

[hex-ci]

@patak-js The problem mentioned in #4939 seems to affect not only Vue 2, but also Vue 3.

[patak-dev]

Oh, I think you should also open an issue here to track it

[Sec32fun32]

#5444