Create and Implement `RequirementChecker` for all entities

[Shadow53]

RequirementChecker defines a function that takes a reference to a Character and determines if it can "take" the given entity, whatever that means in context.

pub trait RequirementChecker {
    check_reqs(&self, c: &Character) -> Result<(), Error> {
        Ok(())
    }
}

... where Error is whatever the custom error type is, probably nebula_error::Error. The above indicates a default implementation that automatically succeeds.