Add rule to omit internal keyword from declarations with internal access control

[calda]

Summary

This PR proposes a new rule to omit the internal keyword from declarations with internal access control.

Autocorrect is implemented in the new redundantInternal SwiftFormat rule, which was added in nicklockwood/SwiftFormat#1486.

// WRONG
internal class Spaceship {
  internal init() { … }

  internal func travel(to planet: Planet) { … }
}

// RIGHT, because internal access control is implied if no other access control level is specified.
class Spaceship {
  init() { … }

  func travel(to planet: Planet) { … }
}

Reasoning

Internal access control is implied for declarations without an explicit access control level, so the internal keyword is redundant.

Please react with 👍/👎 if you agree or disagree with this proposal.

[bachand]

Nice!