feat: emit CSS sourcemap in build mode

[fi3ework]

Description

Resolve #2830, implement CSS sourcemap in build mapping in build mode.
Although this PR is currently a work in progress and is marked as a draft, any early feedback would be appreciated to ensure it's on the right track. The to-do list must be completed before it can be changed from draft status.

• emit linked sourcemap
• combine source map for ?used
• implement inline sourcemap
• mark CSS sourcemap in build mode as experimental
• adding more test case

Additional context

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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 PR Title 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.

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[m5x5]

Thanks for the commit, are you planning on completing it anytime soon? I'm building a package that's going to make use of it. 😁