Add redundantProperty rule

[calda]

Summary

This PR proposes a new rule to avoid defining properties that are then returned immediately. Instead, return the value directly.

Autocorrect support is implemented in the SwiftFormat redundantProperty rule, which as added in nicklockwood/SwiftFormat#1654.

// WRONG
var spaceship: Spaceship {
  let spaceship = spaceshipBuilder.build(warpDrive: warpDriveBuilder.build())
  return spaceship
}

// RIGHT
var spaceship: Spaceship {
  spaceshipBuilder.build(warpDrive: warpDriveBuilder.build())
}

// WRONG
var spaceship: Spaceship {
  let warpDrive = warpDriveBuilder.build()
  let spaceship = spaceshipBuilder.build(warpDrive: warpDrive)
  return spaceship
}

// RIGHT
var spaceship: Spaceship {
  let warpDrive = warpDriveBuilder.build()
  return spaceshipBuilder.build(warpDrive: warpDrive)
}

Reasoning

Property declarations that are immediately returned are typically redundant and unnecessary. Sometimes these are unintentionally created as the byproduct of refactoring. Cleaning them up automatically simplifies the code. In some cases this also results in the return keyword itself being unnecessary, further simplifying the code.

[mannylopez]

Makes sense to me. Can't think of a reason why we wouldn't want to simplify this.