Standarize the changelog messages

AToMPM defines changelog messages to propagate model modifications. For example, adding/deleting/changing nodes or edges. These messages travel from the abstract syntax worker to the concrete syntax worker(s), and from there to the user's browser where they are used to change the canvas.

The issue is that this changelog format is not standarized. They are just dictionaries/arrays sent without a clean structure. This makes it difficult to understand the communication.

This proposal is to make a standard format for the changelog messages. For example, creating classes for each message type, such that the message type `LOADMM` has defined fields and error handling in the getters and setters for each field. This format would then be used on both the client and server side.

In the future, this format could also align with the [Graphical Language Server Platform](https://github.com/eclipse-glsp/glsp) for better interoperability.

No files or tests are named. Start by locating the changelog message definitions and the abstract-syntax, concrete-syntax, and browser communication entry points; document the existing LOADMM shape and define the shared client/server boundary. Done means the message types use a standard format with field validation and handling on both sides.

Written by the indexing model from the issue text.