Implement support for ::before and ::after pseudo elements

[nicoburns]

::before and ::after are quite widely used. And current such content is just missing when rendered with Blitz. Use cases that I'm aware of are:

• Icon fonts such as Font Awesome
• Decorative markers on elements (these are often absolutely positioned)

Some notes:

• ::before and ::after behave as elements inside the node they are targeting, as the first and last children.
• In Blitz should probably be implemented as Option<NodeId> where the node is of type AnonymousBlock (which we might want to rename to PsuedoElement / we might also want to merge with Element).
• They should probably be included in the layout_children of the parent node if possible.
• I'm not quite sure when or how the anonymous nodes should be generated
• We'll also need to clean them up when the parent is dropped (or it's styles change such that it no longer has a ::before or ::after style, but this problem also applies to anonymous blocks so this could be ignored for now (and dealt with along with the other memory leaks).