LibWeb: Don't crash on unknown CSS display types, fall back to inline

This patch also adds CSS::Display::to_string() so we can log the
unimplemented CSS display value (if you have LIBWEB_CSS_DEBUG enabled).

Userland/Libraries/LibWeb/CMakeLists.txt

@@ -28,6 +28,7 @@ set(SOURCES
     CSS/CSSSupportsRule.cpp
     CSS/ResolvedCSSStyleDeclaration.cpp
     CSS/DefaultStyleSheetSource.cpp
+    CSS/Display.cpp
     CSS/Length.cpp
     CSS/MediaList.cpp
     CSS/MediaQuery.cpp