LibWeb: Expose barebones CSSStyleDeclaration to JavaScript

You can now access an element's inline style via Element.style.
The interface is not very capable yet though. :^)

Userland/Libraries/LibWeb/CSS/CSSStyleDeclaration.cpp

@@ -37,4 +37,11 @@ CSSStyleDeclaration::~CSSStyleDeclaration()
 {
 }
 
+String CSSStyleDeclaration::item(size_t index) const
+{
+    if (index >= m_properties.size())
+        return {};
+    return CSS::string_from_property_id(m_properties[index].property_id);
+}
+
 }