Support external URLs with duplicate query parameters

Adding some possibly related tickets.

As some other folks have mentioned, this seems to be an underlying PHP issue, I think with parse_str as called in Url::fromUri:

parse_str($uri_parts['query'], $uri_query);

There's a "solution" on php.net explaining how a "proper_parse_str" might function.

I guess we'd need to...
- decide if Drupal should support duplicate query strings
- (if yes...) write some tests
- implement "proper_parse_str" somewhere
- replace "parse_str" with "proper_parse_str" in Url::fromUri and like... everywhere?

Duplicate: 🐛 Url only outputs the last value of a query parameter Needs work (Also related: ✨ Output multiple values of a query param with repeating param name Needs work )