LibWeb: Add display: math

This is a `<display-inside>` keyword added by the MathML spec, and has
the rough meaning of "display in the default way". It enables the
standard layout rules for each MathML element (and is ignored for
anything that isn't a MathML element).

I believe we'll need an actual MathML formatting context to do the
layout correctly, but we can at least support a couple of elements that
behave the same as HTML ones.

Userland/Libraries/LibWeb/CSS/Parser/Parser.cpp

@@ -3574,6 +3574,8 @@ RefPtr<StyleValue> Parser::parse_display_value(Vector<ComponentValue> const& com
                     return Display::from_short(Display::Short::Grid);
                 case DisplayInside::Ruby:
                     return Display::from_short(Display::Short::Ruby);
+                case DisplayInside::Math:
+                    return Display::from_short(Display::Short::Math);
                 }
             }