### Preflight Checklist

- [X] I have read the [Contributing Guidelines](https://github.com/electron/electron/blob/master/CONTRIBUTING.md) for this project.
- [X] I agree to follow the [Code of Conduct](https://github.com/electron/electron/blob/master/CODE_OF_CONDUCT.md) that this project adheres to.
- [X] I have searched the [issue tracker](https://www.github.com/electron/electron/issues) for a feature request that matches the one I want to file, without success.

### Electron Version

main

### What operating system are you using?

All OSes.

### What arch are you using?

All archs.

### Last Known Working Electron version

N/A

### Expected Behavior

[`ElectronBrowserClient::GetDefaultDownloadDirectory()`](https://github.com/electron/electron/blob/main/shell/browser/electron_browser_client.cc#L1013-L1019) is hardcoded to return `~/Downloads`. However, this might not be the actual downloads directory path (eg: if the user has changed it or on OSes which translate user directory names).

The proper way to get the current user's downloads directory is OS-dependent:
- On Linux it is set in the env var `$XDG_DOWNLOAD_DIR` 
- On Windows it is returned by [`SHGetKnownFolderPath`](https://docs.microsoft.com/en-us/windows/win32/api/shlobj_core/nf-shlobj_core-shgetknownfolderpath) passing in `FOLDERID_Downloads`
- On MacOS it is returned by [`URLForDirectory`](https://developer.apple.com/documentation/foundation/nsfilemanager/1407693-urlfordirectory) passing in `NSDownloadsDirectory`

### Current Behavior

`~/Downloads` is [hardcoded](https://github.com/electron/electron/blob/main/shell/browser/electron_browser_client.cc#L1013-L1019).

shell/browser/electron_browser_client.cc
#endif
  
  
  
  if (process_type != ::switches::kZygoteProcess) {
    base::FilePath user_data_dir;
    if (base::PathService::Get(chrome::DIR_USER_DATA, &user_data_dir))
      command_line->AppendSwitchPath(::switches::kUserDataDir, user_data_dir);
  }
  if (process_type == ::switches::kUtilityProcess ||
      process_type == ::switches::kRendererProcess) {
    
    static const char* const kCommonSwitchNames[] = {
        switches::kStandardSchemes,      switches::kEnableSandbox,
        switches::kSecureSchemes,        switches::kBypassCSPSchemes,
        switches::kCORSSchemes,          switches::kFetchSchemes,
        switches::kServiceWorkerSchemes, switches::kEnableApiFilteringLogging,
        switches::kStreamingSchemes};
    command_line->CopySwitchesFrom(*base::CommandLine::ForCurrentProcess(),
                                   kCommonSwitchNames,
                                   base::size(kCommonSwitchNames));
  }
  if (process_type == ::switches::kRendererProcess) {
#if defined(OS_WIN)
    
    PWSTR current_app_id;
    if (SUCCEEDED(GetCurrentProcessExplicitAppUserModelID(&current_app_id))) {
      command_line->AppendSwitchNative(switches::kAppUserModelId,
                                       current_app_id);
      CoTaskMemFree(current_app_id);