Add Windows support to url_launcher plugin

[kjk]

The code is as simple as:

Future<ProcessResult> launchURL(String url) {
  url = url.replaceAll("&", "^&");
  return Process.run("start", [url], runInShell: true);
}

url = url.replaceAll("&", "^&"); is needed because runInShell runs the command as equivalent of cmd.exe /c "start ${url}" and & is interpreted by cmd.exe and needs to be escaped with ^.

[stuartmorgan]

Duplicate of #41721

[github-actions]

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