Add constants for known ecs fields for code efficiency when you log

Hi,
I'd like to have the ability to contribute to the project. It would be the first time I do it, so I am not sure if I can create a branch or anything. Would that be possible?

I'd like juste to create a Constants.java class that would have all the ECS fields so it would be accessible from any project and that would be more efficient.

Plus, it would be more easy to look at which fields already exist.