Add starlark output format

[fmeum]

TODO:

• Add support for Pair (tuple)
• Insert empty lines around top-level statements
• Sort keyword arguments
• Find a way to annotate classes with a Bazel load label, collect the labels for all classes in the module and emit them at the top

[HT154]

Hi there! We generally encourage renderer development to be done in Pkl code to avoid increasing the API surface and maintenance cost of the core language. Pkl code may subclass ValueRenderer and directly implement its methods. This is how the TOML renderer is implemented: https://github.com/apple/pkl-pantry/tree/main/packages/pkl.toml. It would be much easier for us to accept a contribution of that nature.