feat: Support a flag to preserve blank lines during formatting

Currently, `yamlfmt` preserves original blank lines only when `retain_line_breaks: true` is set in a configuration file. It would be great to have a direct command-line option, like `--preserve-blank-lines`, to achieve the same behavior without relying on a config file.

To make this even more user-friendly, could we introduce a simple shorthand as well? Since no existing shorthand uses `-p`, how about `-p` as a shortcut for `--preserve-blank-lines`? This would give users a quick and easy way to keep their original blank lines intact, which is especially helpful for ad-hoc formatting tasks or integrating `yamlfmt` into scripts and CI workflows.