fix tarcking-vector image in dark-mode

Since whatwg/whatwg.org#444 *tracking-vector.svg* is now darkmode-aware, i.e. its fill color will be dependent on the user's darkmode settings. However from whatwg/whatwg.org#439 `<img>` tags pointing at svg images that don't have a `"darkmode-aware"` class are given a white background by default. This makes the tracking-vector image fully white when seen in dark-mode.
This PR aims at fixing it by adding the class on the generated `<img>` element.

Note that I'm not entirely sure this is the only source for this image in all the specs, but I tested it with html-build where it already handles a bunch.

src/wattsi.pas

@@ -1692,6 +1692,7 @@    TCrossReferences = record
                                               'alt', '(This is a tracking vector.)',
                                               'width', '46',
                                               'height', '64',
+                                              'class', 'darkmode-aware',
                                               'crossorigin', ''])]);
             (Node.ParentNode as TElement).ReplaceChild(TempElement, Node);
             Node.Free();