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/Bindings/CSSNamespace.h

@@ -0,0 +1,28 @@
+/*
+ * Copyright (c) 2021, Sam Atkins <atkinssj@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include <LibJS/Runtime/Object.h>
+#include <LibWeb/CSS/Serialize.h>
+
+namespace Web::Bindings {
+
+// The `CSS` namespace object in IDL. https://www.w3.org/TR/cssom-1/#namespacedef-css
+class CSSNamespace final : public JS::Object {
+    JS_OBJECT(CSSNamespace, JS::Object)
+
+public:
+    explicit CSSNamespace(JS::GlobalObject&);
+    virtual void initialize(JS::GlobalObject&) override;
+    virtual ~CSSNamespace() override;
+
+private:
+    JS_DECLARE_NATIVE_FUNCTION(escape);
+    JS_DECLARE_NATIVE_FUNCTION(supports);
+};
+
+}