Add iteration support to RmlUi data model handles

The data model handle returned from RmlUi when creating a data model cannot be iterated over. This is because the handle is an empty table with a metatable to detect changes. A pairs function needs to be set on the metatable so that the handle can have iteration. See the example provided by sol:

Locate the RmlUi data model handle creation path and inspect how its empty table metatable detects changes. Read the linked sol pairs.cpp example for the relevant iteration behavior. Done means the returned handle supports iteration without breaking change detection; verify with the project's existing data-model tests if available.

Written by the indexing model from the issue text.