LibWeb: Implement the math-depth CSS property

This one is a bit fun because it can be `add(<integer>)` or `auto-add`,
but children have to inherit the computed value not the specified one.
We also have to compute it before computing the font-size, because of
`font-size: math` which will be implemented later.

Userland/Libraries/LibWeb/Forward.h

@@ -137,6 +137,7 @@ class LengthPercentage;
 class LengthStyleValue;
 class LinearGradientStyleValue;
 class ListStyleStyleValue;
+class MathDepthStyleValue;
 class MediaFeatureValue;
 class MediaList;
 class MediaQuery;