### 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
base::CommandLine* command_line,
    int process_id) {
  
  {
    base::ThreadRestrictions::ScopedAllowIO allow_io;
    base::FilePath child_path;
    base::FilePath program =
        base::MakeAbsoluteFilePath(command_line->GetProgram());
#if defined(OS_MAC)
    auto renderer_child_path = content::ChildProcessHost::GetChildPath(
        content::ChildProcessHost::CHILD_RENDERER);
    auto gpu_child_path = content::ChildProcessHost::GetChildPath(
        content::ChildProcessHost::CHILD_GPU);
#if BUILDFLAG(ENABLE_PLUGINS)
    auto plugin_child_path = content::ChildProcessHost::GetChildPath(
        content::ChildProcessHost::CHILD_PLUGIN);
#endif
    if (program != renderer_child_path && program != gpu_child_path
#if BUILDFLAG(ENABLE_PLUGINS)
        && program != plugin_child_path
#endif
    ) {
      child_path = content::ChildProcessHost::GetChildPath(
          content::ChildProcessHost::CHILD_NORMAL);
      CHECK_EQ(program, child_path)
          << "Aborted from launching unexpected helper executable";
    }