LibWeb: Add the default user agent MathML stylesheet

We now apply MathML's default user agent style sheet along with other
default styles. This sheet is not mixed in with the other styles in
CSS/Default.css because it is a namespaced stylesheet and so has to
be its own sheet.

Userland/Libraries/LibWeb/HTML/Parser/HTMLParser.cpp

@@ -35,6 +35,7 @@
 #include <LibWeb/HighResolutionTime/TimeOrigin.h>
 #include <LibWeb/Infra/CharacterTypes.h>
 #include <LibWeb/Infra/Strings.h>
+#include <LibWeb/MathML/TagNames.h>
 #include <LibWeb/Namespace.h>
 #include <LibWeb/SVG/TagNames.h>