LibWeb: Make grid containers be Layout::Box

Grid containers were incorrectly represented as BlockContainer before.
Furthermore, GridFormattingContext had a bogus inheritance relationship
with BlockFormattingContext.

This patch brings our architecture closer to spec by making grid
containers be plain boxes and making GFC not inherit from BFC.

Userland/Libraries/LibWeb/Layout/FormattingContext.h

@@ -21,6 +21,7 @@ public:
         Block,
         Inline,
         Flex,
+        Grid,
         Table,
         SVG,
     };