Support multi-document dumping

[ingydotnet]

https://clojurians.slack.com/archives/C042XAQFCCU/p1728859798406169

A :dump-all for generate-stream might be a nice counterpart to parse-string's :load-all option.

SnakeYAML should have what you need to do this simply and correctly.

[ingydotnet]

See https://github.com/snakeyaml/snakeyaml/blob/4f5f973c4a27ab3fa0c42dd0eb491bca06bb3df7/src/main/java/org/yaml/snakeyaml/Yaml.java#L265-L269