LibWeb: Added simple parsing for (at)import rules

For now, the parsing is very crude, and parses only the document url.
The meta queries for the import are ignored.

Userland/Libraries/LibWeb/CSS/Parser/CSSParser.h

@@ -27,6 +27,7 @@
 #pragma once
 
 #include <AK/NonnullRefPtr.h>
+#include <AK/String.h>
 #include <LibWeb/CSS/StyleSheet.h>
 
 namespace Web::CSS {
@@ -38,6 +39,8 @@ public:
 
     bool in_quirks_mode() const;
 
+    URL complete_url(const String&) const;
+
 private:
     const DOM::Document* m_document { nullptr };
 };