Support multiline commands in comments

Sometimes, I need to write long commands that I don't want to show the user. However, `mdsh` currently supports only single-line commented-out commands ([link](https://github.com/zimbatm/mdsh?tab=readme-ov-file#commented-out-commands)). ## Example For example, I created a code block with a command that shouldn't be executed by `mdsh`: https://github.com/nix-community/cache-nix-action/blob/5ccd9ce507c7199b6ea3ca13797f763f69a4b7c6/README.md?plain=1#L335 and the command that should be executed: https://github.com/nix-community/cache-nix-action/blob/5ccd9ce507c7199b6ea3ca13797f763f69a4b7c6/README.md?plain=1#L340 The latter command is quite long and it'd be convenient if I could rewrite it as a multiline one.