LibWeb: Add XMLHttpRequest::open() overload

This adds the XMLHttpRequest::open(String, String, bool, String, String)
overload.

The following FIXMEs has been implemented:
- If method is not a method, then throw a "SyntaxError" DOMException.
- If the username argument is not null, set the username given parsedURL
  and username.
- If the password argument is not null, set the password given parsedURL
  and password.
- Set this’s synchronous flag if async is false; otherwise unset this’s
  synchronous flag.

Spec comments has also been updated.

Userland/Libraries/LibWeb/XHR/XMLHttpRequest.idl

@@ -27,6 +27,7 @@ interface XMLHttpRequest : XMLHttpRequestEventTarget {
     attribute XMLHttpRequestResponseType responseType;
 
     undefined open(DOMString method, DOMString url);
+    undefined open(ByteString method, USVString url, boolean async, optional USVString? username = {}, optional USVString? password = {});
     undefined setRequestHeader(DOMString name, DOMString value);
     undefined send(optional USVString body = {});