fix(hast)!: unwrap a Root node and rename property class to className

[tenjo-t]

Description

• fix Is it Appropriate for @shikijs/rehype to Output a HAST Root Node? #1033
• rename property class to className

hast uses className instead of class. Utilities such as hast-util-classnames handle only className.

The HTML attributes class and for respectively become className and htmlFor in alignment with the DOM. No other attributes gain different names as properties, other than a change in casing.
https://github.com/syntax-tree/hast?tab=readme-ov-file#propertyname

Linked Issues

#1033

Additional context

processNode was modified in #896.

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[antfu]

I think this would be a breaking change that we need to pending it until the next major