Engine module structure: consider grouping by "node type" or other client-facing concept

As suggested in [this comment](https://github.com/getodk/web-forms/pull/277#discussion_r1920533235) on #277.

I'm opening this as a stub issue for now so I can reference it in response to that comment. We can fill out details as appropriate.

Start by reading the linked discussion on pull request #277 and inspecting the current engine module structure. No files or tests are named, so first determine which client-facing concepts the modules represent and agree on the intended grouping. Done means the structure and scope are clearly defined for implementation.

Written by the indexing model from the issue text.