LibHTML: Implement CSS text-align: left/center/right

This was easier than I imagined; we just shift each line box to the
left based on the alignment and the remaining space on each line. :^)

Libraries/LibHTML/CSS/StyleValue.h

@@ -14,6 +14,9 @@ namespace CSS {
 enum class ValueID {
     Invalid,
     VendorSpecificLink,
+    Center,
+    Left,
+    Right,
 };
 }