Add emptyExtension rule

[mannylopez]

Summary

This PR adds rule emptyExtension to remove empty extensions that define no properties, functions, or conformances.

// WRONG: The first extension is empty and redundant.
extension Planet {}

extension Planet: Equatable {}

// RIGHT: Empty extensions that add a protocol conformance aren't redundant.
extension Planet: Equatable {}

Reasoning

Improves readability since the code has no effect and should be removed for clarity.

[calda]

Good internal support for this change, thanks @mannylopez!