`extern_class!` shorthand for property access

[madsmtm]

Some classes have a lot of simple properties; it may be interesting to have an easier way of accessing these?

Examples:

• UIColor has a lot of class properties that return a specific UIColor instance
• UIView has a lot of getter/setter properties
  • Difficult because some cases might use &mut self for mutation, while others might use &self
• The x -> isX translation for BOOL properties can be kinda confusing, and would be nice to handle automatically

See also https://github.com/madsmtm/objc2/pull/161#issuecomment-1166560468.