feat: allow multiple documents in `std.parseYaml`

[alexatcanva]

Heya Team :wave:

While trying to give sjsonnet a whirl, we noticed a few of our std.parseYaml calls were failing if they included multi doc YAML. (deliminated by the old ---).

This PR allows the yamlToJson function to parse these files/YAML streams and return the appropriate JSON string.

I'm a first time contributor to this repo, and furthermore it's my first time writing Scala.

Please do let me know if there's a better way to accomplish this, I'm more than happy to work through any of the feedback I receive!

Thanks!

[alexatcanva]

Heya @lihaoyi-databricks 👋

Is there any chance I could please get a review on this one? ❤️ Thanks!