feat(css): Implement codegen for css

[kdy1]

swc_css_ast:

• Remove unused tokens.
• SubclassSelector::PseudoClass => SubclassSelector::Pseudo.
• Fix AttrSelectorOp
• AttrSelector.value: Option<Text> => Option<Str>.

swc_css_codegen:

• Implement code generator.

swc_css_parser:

• Implement Parse<Stylesheet> for the parser.
• Fix parsing of pseudo selectors.
• Fix parsing of unknown at-rules.

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Closes #16