This is done by adding a new entry SelectionType::eHighlight.
In contrast to the existing selection types, highlight selections can occur more than once
and are identified by their highlightName (i.e., ::highlight(foo) in css).
This made some changes necessary to nsFrameSelection,
which now uses an additional hash map of highlight selections,
as well as to several methods dealing with getting style properties for selected text
in nsTextFrame, which now in addition to the SelectionType enum need the highlight name
as well.
Depends on D163008