LibWeb: Add interface for 'concept-url-parser'

This does not implement extra functionality on top of the basic parser,
but allows multiple places in LibWeb to call the 'correct' interface for
when it is fully implemented.

Userland/Libraries/LibWeb/URL/URL.h

@@ -77,4 +77,7 @@ private:
 HTML::Origin url_origin(AK::URL const&);
 bool host_is_domain(StringView host);
 
+// https://url.spec.whatwg.org/#concept-url-parser
+AK::URL parse(StringView input, Optional<AK::URL> const& base_url = {});
+
 }