LibWeb: Obey CSS aspect-ratio property during layout

Calculate a "preferred aspect ratio" based on the value of
`aspect-ratio` and the presence of a natural aspect ratio, and use that
in layout.

This is by no means complete or perfect, but we do now apply the given
aspect-ratio to things.

The spec is a bit vague, just saying to calculate sizes for
aspect-ratio'ed boxes the same as you would for replaced elements. My
naive solution here is to find everywhere we were checking for a
ReplacedBox, and then also accept a regular Box with a preferred aspect
ratio. This gets us pretty far. :^)

https://www.w3.org/TR/css-sizing-4/#aspect-ratio-minimum is not at all
implemented.

Userland/Libraries/LibWeb/Layout/Box.h

@@ -41,6 +41,10 @@ public:
     void set_natural_height(Optional<CSSPixels> height) { m_natural_height = height; }
     void set_natural_aspect_ratio(Optional<float> ratio) { m_natural_aspect_ratio = ratio; }
 
+    // https://www.w3.org/TR/css-sizing-4/#preferred-aspect-ratio
+    Optional<float> preferred_aspect_ratio() const;
+    bool has_preferred_aspect_ratio() const { return preferred_aspect_ratio().has_value(); }
+
     virtual ~Box() override;
 
     virtual void did_set_content_size() { }