feat(validation): add Closure rule

[MohammadAlhallaq]

#1775

[innocenzi]

A few things:

• The name of the attribute/rule should be something more explicit that "Closure"
• There should be a test with a class and a property actually using the rule
• As I said in the issue, I really don't think this is a good pattern that we should promote by supporting it

[MohammadAlhallaq]

A few things:

• The name of the attribute/rule should be something more explicit that "Closure"
• There should be a test with a class and a property actually using the rule
• As I said in the issue, I really don't think this is a good pattern that we should promote by supporting it

Yeah, I was planning to revisit it today, I’ll take care of the first two points, and we can wait for brendt's review.