Write back to Yaml similar to writing back to Json

[afsalthaj]

We have a zio.config.typesafe package that can write back any config description to Json. We need the similar functionality for yaml, as currently zio-config can only read yaml, and cannot write back to yaml.

This will also turn out to be a simple tool for users to convert json to its yaml representation

[afsalthaj]

Here are the example steps required to write a config back to its source.

1. For typesafe-hocon config, we built a function that goes from propertyTree to `com.typesafe.config.Config,

https://github.com/zio/zio-config/blob/master/typesafe/shared/src/main/scala/zio/config/typesafe/TypesafeConfigSource.scala#L226

2. Similary we need a function that goes from PropertyTree to YamlConfig.

This means you can imagine every config in various libraries can be turned to a PropertyTree.

[l0kr]

I'll work on this one

[afsalthaj]

Thanks @co0lster