xhr: impossible to open relative URL

[krichprollsch]

const req = new XMLHttpRequest();                                       
req.open('GET', 'index.html');

SyntaxError: TODO message

[krichprollsch]

This issue is blocked by lightpanda-io/zig-js-runtime#202 b/c we need to access to the base URL from document (or somewhere similar).

[karlseguin]

Relative XHR URLs are working.