Added nvda-mathcat submodule (#18610)
This PR adds MathCAT assets under include/nvda-mathcat as a submodule.
We are opening this as a separate PR to create this submodule before the
main PR (#18323) is merged for the MathCAT integration project so that
we will have access to this submodule for testing the GitHub Actions
workflow in [the nvda-mathcat
repo](https://github.com/nvaccess/nvda-mathcat) that automatically
fetches assets from the latest MathCAT release.
