LibWeb+LibSyntax: Implement nested syntax highlighters

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.

Userland/Applications/Spreadsheet/CellSyntaxHighlighter.cpp

@@ -31,7 +31,7 @@ void CellSyntaxHighlighter::rehighlight(const Palette& palette)
             false,
             false,
         },
-        nullptr,
+        (u64)-1,
         false);
 
     if (m_cell && m_cell->exception()) {
@@ -47,7 +47,7 @@ void CellSyntaxHighlighter::rehighlight(const Palette& palette)
                     false,
                     false,
                 },
-                nullptr,
+                (u64)-1,
                 false });
     }
     m_client->do_update();