Add unusedPrivateDeclaration rule

[mannylopez]

Summary

This PR adds rule unusedPrivateDeclaration to remove unused private and fileprivate properties, functions, and typealiases.

// WRONG: Includes private declarations that are unused
struct Planet {
  var ageInBillionYears: Double {
    ageInMillionYears / 1000
  }

  private var ageInMillionsOfYears: Double
  private typealias Dependencies = UniverseBuilderProviding // unused
  private var mass: Double // unused
  private func distance(to: Planet) { } // unused
}

// RIGHT
struct Planet {
  var ageInBillionsOfYears: Double {
    ageInMillionYears / 1000
  }

  private var ageInMillionYears: Double
}

Reasoning

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

[calda]

Excited about this one, nice work @mannylopez!