Url only outputs the last value of a query parameter

Re-rolled #40 for 9.4.12

Remade the patch from #48

Re-rolled #40 for 9.4.12

Re-rolled #49 for 10.2.3

It looks like there's an unanswered question of ticket scope: are we only interested in external URLs, or should we also deal with internal? Personally I'd suggest that if we can't simply solve both, then it makes sense to at least support external URLs along the lines of #31 and if we want to handle internal links do that in a separate ticket. The only issue with #31 that I can see is the slight WTF you'd get from adding an entirely separate query param to such an external URL: I don't think you'd expect that to change how the duplicate query parameters are handled. ie...

// Assuming we're using a solution that only implements #31.
\Drupal\Core\Url::fromUri('https://example.com/page?tag=one&tag=two')->toString();
// Returns https://example.com/page?tag=one&tag=two
\Drupal\Core\Url::fromUri('https://example.com/page?tag=one&tag=two', ['query' => ['foo' => 'bar']])->toString();
// Returns https://example.com/page?tag=two&foo=bar

@carolpettirossi could you give any extra detail on why/how you're using local links with duplicate query parameter names? Thanks!

I can confirm patch #51 works on Drupal 10.3.

Hello, I have tried using the patch at #51 on a 10.2.11 project.
This patch applies and retains the duplicate query parameter, following the proposed solution pattern (?tag[0]=one&tag[1]=two) but with an error clicking the link: A client error happened

```
Symfony\Component\HttpKernel\Exception\BadRequestHttpException: Input value "keyword" contains a non-scalar value. in Symfony\Component\HttpKernel\HttpKernel->handle() (line 83 of /var/www/html/vendor/symfony/http-kernel/HttpKernel.php).
```

The link field is using an internal url going to a filtered by keyword search.
`/collection/search?keyword%5B0%5D=Ryakuga&keyword%5B1%5D=haya-oshie`
When there are more than one of the same query parameter, only the last value is outputting.

I've tried the same patch on a 10.3.10 project, again patch applies, retains the duplicate parameter with the pattern of (?tag[0]=one&tag[1]=two),however clicking the link strips off all parameters completely.

Wondering if anyone else has seen anything similar (also trying to dig into if this is just something in our internal setup).