Update to godot-cpp v10 and save demo in Godot 4.5

[dsnopek]

This modernizes this example by:

• Updating to godot-cpp v10 (currently master, I'll update for a release once we have one) and all follow-up changes
• Opening the demo in Godot 4.5 and running the "Upgrade Project Files..." tool
• Removes the GDScript code and autoload to register the XRInterface - this is no longer needed in Godot 4.5+
• Tweaking the GitHub Actions config until it runs :-)

And a couple of other minor things:

• Switching from header guards to #pragma once
• Removing all unneeded includes
• Adding the build object files to the .gitignore
• Adding a build_profile.json to speed up GitHub Actions

I'm going to mark this one as DRAFT until there's a godot-cpp v10 release (which should be in a few days)