LibWeb: Add CSS.escape() JS function

This is the `CSS` namespace defined in IDL here:
https://www.w3.org/TR/cssom-1/#namespacedef-css , not to be confused
with our `Web::CSS` namespace. Words are hard.

`CSS.escape()` lets you escape identifiers that can then be used to
create a CSS string.

I've also stubbed out the `CSS.supports()` function.

Userland/Libraries/LibWeb/CMakeLists.txt

@@ -1,4 +1,5 @@
 set(SOURCES
+    Bindings/CSSNamespace.cpp
     Bindings/CSSRuleWrapperFactory.cpp
     Bindings/CSSStyleDeclarationWrapperCustom.cpp
     Bindings/EventListenerWrapper.cpp
@@ -14,6 +15,7 @@ set(SOURCES
     Bindings/WindowObject.cpp
     Bindings/Wrappable.cpp
     Crypto/Crypto.cpp
+    CSS/Serialize.cpp
     CSS/CSSConditionRule.cpp
     CSS/CSSGroupingRule.cpp
     CSS/CSSImportRule.cpp