LibWeb: Parse CSS background-position property

This is done a bit differently from other properties: using a
TokenStream instead of just a Vector of ComponentValues. The reason for
this is, we can then use call the same function when parsing the
`background` shorthand. Otherwise, we would have to know in advance how
many values to pass down, which basically would involve duplicating the
`background-position` parsing code inside `background`.

The StyleValue is PositionStyleValue, since it represents a
`<position>`: https://www.w3.org/TR/css-values-4/#typedef-position
Unfortunately, background-position's parsing is a bit different from
`<position>`'s, (background-position allows 3-value syntax and
`<position>` doesn't) so we'll need to come back and write a different
parsing function for that later.

Userland/Libraries/LibWeb/Forward.h

@@ -53,6 +53,7 @@ class MediaQueryList;
 class MediaQueryListEvent;
 class NumericStyleValue;
 class OverflowStyleValue;
+class PositionStyleValue;
 class PropertyOwningCSSStyleDeclaration;
 class Screen;
 class Selector;