[feat] Add abstract syntax tree modification and write to .pkl file capabilities to language bindings

## Purpose

I'd like to create a wizard application for managing kubernetes manifests, which reads and writes to a set of .pkl files. At the moment it's possible to read from a .pkl file and convert pkl types and primitives to language types and primitives, but I don't see any ability to then write and modify the underlying .pkl files.

## Impact

This feature would open up new applications for Pkl in automation contexts. Many programs have wizards for example that create configuration files for users, or have the ability to maintain a configuration file as a feature. This addition to the language bindings would allow Pkl to be used in these contexts, increasing the number of appropriate use cases and adoption of the language.

Start by reviewing the existing language-binding support for reading .pkl files and converting Pkl types and primitives into language types. Define the scope for AST modification and writing the underlying .pkl files, then identify how completion will be verified for both capabilities.

Written by the indexing model from the issue text.