Support yaml marshal/unmarshal

For all the same reasons the `json` module was added to starlark.
Note there's an implementation over here for the Go implementation of starlark https://github.com/qri-io/starlib/tree/master/encoding/yaml but I doubt that's useful in any way.

The motivation is that the `pnpm` tool needs to be integrated as a repository rule, and the data needed to fetch packages in in yaml: https://pnpm.io/git#lockfiles

As a workaround we're using the `yq` toolchain to translate yaml to json, which starlark does know how to read: https://docs.aspect.build/aspect-build/bazel-lib/v0.9.6/docs/yq-docgen.html#yq
however it's difficult in a Bazel repository rule to access a tool provided by a toolchain, and impossible under bzlmod (https://github.com/bazelbuild/bazel/issues/14445 - one module extension fetches a `yq` binary but another module extension wants to run it)

Start by reading the existing `json` module in Starlark and the linked Go Starlark YAML implementation. Define the required marshal/unmarshal behavior from the pnpm lockfile use case, then verify that YAML data can be consumed by repository rules and module extensions without the `yq` workaround.

Written by the indexing model from the issue text.