Fix OUTPUT DIRECTORY button on WSL

[przemeksulecki]

This is a PR regarding the OUTPUT DIRECTORY button located in the player's video settings.

When we run npm on WSL (Windows Subsystem for Linux) then Motion Canvas tries to execute:
xdg-open <absolute-path-to-output-dir>

In the case of WSL it won't work, so we can do something like this:
bash -c cd <absolute-path-to-output-dir> && explorer.exe .

[aarthificial]

Since the explorer variable no longer makes sense with WSL, I recommend we rewrite this function a bit to avoid this confusing explorer === 'bash' check at the end:

export function openInExplorer(file: string) {
  let command: string | null = null;
  let args = [file];
  const os = platform();

  switch (os) {
    case 'win32':
      command = 'explorer';
      break;
    case 'linux':
      if (isRunningOnWSL()) {
        command = 'bash';
        args = ['-c', `cd ${file} && explorer.exe .`];
      } else {
        command = 'xdg-open';
      }
      break;
    case 'darwin':
      command = 'open';
      break;
  }

  if (command) {
    spawn(command, args, {detached: true}).unref();
  } else {
    console.warn(`Unsupported OS: ${os}`);
  }
}

[przemeksulecki]

I did force-push because I initially forgot to add --fixup flag during committing.