Support final properties in entity templates

[ProfDiesel]

Compilation fix for Haxe 5.

[player-03]

I'm worried that this would be misleading. Entity templates are just syntax sugar, and can't prevent a component from being overwritten. Even if we declare the property as (get, never) and don't include a setter, the user can still modify the component by calling add().

If it's just meant as a suggestion or reminder not to modify the component, that's fair, but I'd want to go about that in a less-misleading way.

If you got an error when you typed final and wanted to fix the error, we could do a quick access.remove(AFinal). But again, final is misleading, so perhaps it should remain an error.

[player-03]

I've added an error message to the Haxe 4 version, for consistency. If you'd like to make the case that we should support final components, get back to me and I can reopen this.