Fix the error with including a non-existent file
Fix the error with including a non-existent file

Not all machines have oh-my-zsh installed, since not all machines need
it, and conditional loading is added to avoid errors.

```
xor at ant in ~/work/dotfiles on  main [$]
❯ zsh
/home/xor/.zshrc:source:85: no such file or directory: /home/xor/.oh-my-zsh/oh-my-zsh.sh
```

By-Vasiliy/dotfiles,By-Vasiliy/dotfiles