Add MustFirstEntity

[m110]

Note: shouldn't these methods use Entry instead of Entity? Like FirstEntry? Since it returns an entry. 🤔

[yohamta]

@m110 Thank you very much for adding the helpful function 😄 Sorry for the delay in responding to your pull request. I've been overwhelmed with notifications on GitHub and I inadvertently missed your request for a few days.

[yohamta]

Yeah, I agree. I think the name should be using Entry instead of Entity. I think we can add new methods with proper names and deprecate the old method name 🙂

On second thought, one concern is that since ECS stands for Entity, users might expect it to have a FirstEntity() method as well, so it might be a bit confusing. I think naming it just First() might be good 🤔