pkl Writer

I would like to be able to evaluate existing pkl files and also write changes.

For my use case, I want to provide an UI for a pkl-based configuration. I then want write back the changes the user has made to the original pkl file.

The issue names no files, tests, or entry points. Start by surveying the repository's existing pkl evaluation API and its handling of configuration data, then define how a writer would preserve and update an existing pkl file. Done means users can evaluate an existing file, modify values through the Go bindings, and write those changes back.

Written by the indexing model from the issue text.