LibWeb: Add a very naive Layout::FlexFormattingContext :^)

This is very dumb and only lays out its child boxes on a horizontal
line with their shrink-to-fit widths.

You have to start somewhere! :^)

Userland/Libraries/LibWeb/CMakeLists.txt

@@ -152,6 +152,7 @@ set(SOURCES
     Layout/ButtonBox.cpp
     Layout/CanvasBox.cpp
     Layout/CheckBox.cpp
+    Layout/FlexFormattingContext.cpp
     Layout/FormattingContext.cpp
     Layout/FrameBox.cpp
     Layout/ImageBox.cpp