LibWeb: Implement @supports rule :^)

The main thing missing is that we don't serialize the supports clause,
but for actually using a `@supports (something: cool) {}` rule in CSS,
it works!

Userland/Libraries/LibWeb/CMakeLists.txt

@@ -25,6 +25,7 @@ set(SOURCES
     CSS/CSSStyleDeclaration.cpp
     CSS/CSSStyleRule.cpp
     CSS/CSSStyleSheet.cpp
+    CSS/CSSSupportsRule.cpp
     CSS/ResolvedCSSStyleDeclaration.cpp
     CSS/DefaultStyleSheetSource.cpp
     CSS/Length.cpp