Fix function-url-quotes autofix for multiple url()

[ybiquitous]

Which issue, if any, is this issue related to?

Closes #6710

Is there anything in the PR that needs further explanation?

This pull request rewrites the rule implementation to use the postcss-value-parser AST node rather than string-based indexes.

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Latest commit: 9a103c1

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[jeddy3]

If url-prefix() is only used by the deprecated @document, we can remove checking url-prefix() from the rule. It'd be a non-breaking change, making the rule more permissive.

[ybiquitous]

@jeddy3 Thanks for the feedback. Indeed, when seeing the browser compatibility table of @document, it seems only outdated Firefox versions support url-prefix(), and so it seems safe to remove the function from the test cases.

[jeddy3]

Fantastic, thank you!

It's a nice refactor and fix.

[ybiquitous]

Thanks for the quick review!