LibPDF: Implement ZapfDingbats clause of the adobe glphy list algorithm

Liberation Sans still doesn't have the vast majority of the
Zapf Dingbats glyphs, but now we map the Zapf Dingbats names to good
unicode values.  So we only need to use a different font and all should
work.  (And Liberation Sans has _some_ of the glyphs, like 13 of the
223.) And we now render empty squares instead of wrong glyphs for the
ones we don't have.

I haven't seen any PDFs using ZapfDingbats in the wild, but they
probably exist somewhere.
(Tests/LibPDF/standard-14-fonts.pdf is a synthetic PDF using it.)

Userland/Libraries/LibPDF/Fonts/AdobeGlyphList.h

@@ -10,6 +10,6 @@
 
 namespace PDF {
 
-Optional<u32> glyph_name_to_unicode(StringView);
+Optional<u32> glyph_name_to_unicode(StringView, bool is_zapf_dingbats);
 
 }