Support for `::highlight()` pseudo-element

[hejhi]

Would love to see support for the ::highlight() pseudo-element, which appears to be not supported at the moment.

[lucasweng]

Hi @devongovett, Do you think we need to validate the allowable properties for ::highlight(), perhaps by implementing another declaration parser that integrates with the current one, or is basic support in the SelectorParser sufficient?

[devongovett]

ehh not sure it's worth the extra validation. we could just add it here https://github.com/parcel-bundler/lightningcss/blob/master/src/selector.rs#L259