fix(css): convert map returned by vite:css transform to SourceMap (fixes #4939)

[dominikg]

(fixes #4939)

Description

the map returned by postcss can be a SourceMapGenerator. Use a function to check and call toJSON() on it.
additionally throw an error if the returned map is not something we can pass on as part of the transform result.

An alternate solution would be to roll back the offending commit as this change would not lead to satisfactory sourcemap support on it's own and getting it done isn't a short term endeavor

Additional context

I wasn't able to come up with a testcase for this due to that fact that this map isn't turning up in any results. vite css sourcemaps are completely broken at the moment, especially for bundled and minified output a lot of things need to be added.

1. capture and merge maps in css bundling
2. pass on sourcemap option to esbuild minifier (careful, esbuild has different options, eg. no "hidden")
3. output css maps on dev somehow, maybe even for css turned to js modules?
4. ... ?

there is an old enhancement ticket about it #2830 maybe it's time to revisit that someday

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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.
• Read the Pull Request Guidelines and follow the Commit Convention.
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[patak-dev]

@dominikg thanks for digging into this. We are aiming to start the beta period for 2.6 (probably next Monday). And then there will be likely a week for the community to test it. If you think you or others can work on this to try to support it and use the beta for testing, I think it is good. If not, IMO better we revert and do this more calmly for 2.7

[dominikg]

revert PR is here: #4951