LibWeb: Support highlight pseudo-elements

[tjk113]

Highlight pseudo-elements control styling for selected/highlighted elements. The spec only allows for limited, non-layout-altering properties to be applied to these selectors. The allowed properties are as follows:

• color
• background-color
• text-decoration and its associated properties (including text-underline-position and text-underline-offset)
• text-shadow
• stroke-color
• fill-color
• stroke-width

Supported pseudo-elements:

• ::selection
  Applies to elements that have been selected by the user. Currently, this selector does get parsed, but goes unused, as the selection colors are hard-coded to their CSS::SystemColor defaults. See this discussion in the SerenityOS Discord server for more information regarding implementation.
• ::target-text
  Applies to a page's targeted text fragment. Text fragments are currently unimplemented in Ladybird.
• ::spelling-error
  Applies to text that has been reported by the browser to be misspelled. This requires a spellcheck engine to be integrated into Ladybird.
• ::grammar-error
  Applies to text that has been reported by the browser to be grammatically incorrect. This requires a grammar checking engine to be integrated into Ladybird.