Previously, HTMLToken would expose the Vector<Attribute> directly to
its users. In preparation for a future change, all users now use
implementation-agnostic APIs which do not expose the Vector directly.
And use them to highlight javascript in HTML source.
This commit also changes how TextDocumentSpan::data is interpreted,
as it used to be an opaque pointer, but everyone stuffed an enum value
inside it, which made the values not unique to each highlighter;
that field is now a u64 serial id.
The syntax highlighters don't need to change their ways of stuffing
token types into that field, but a highlighter that calls another
nested highlighter needs to register the nested types for use with
token pairs.
This changes the HTML SyntaxHighlighter to conform to the now-fixed
rendering of syntax highlighting spans in GUI::TextEditor. It also
avoids emitting tokens if they have a zero or negative length.
This fixes a bug where single-character tokens were not highlighted
properly.
