Add rule to prefer shorthand `if let` optional unwrapping syntax

[calda]

Summary

This PR proposes a new rule to prefer the shorthand if let optional unwrapping syntax added in Swift 5.7:

Omit the right-hand side of the expression when unwrapping an optional property to a non-optional property with the same name.

// WRONG
if
  let galaxy = galaxy,
  galaxy.name == "Milky Way"
{ … }

guard
  let galaxy = galaxy,
  galaxy.name == "Milky Way"
else { … }

// RIGHT
if
  let galaxy,
  galaxy.name == "Milky Way"
{ … }

guard
  let galaxy,
  galaxy.name == "Milky Way"
else { … }

Reasoning

We are adopting Swift 5.7 soon, so we should adopt guidance for its new language features.

As discussed in the Motivation section of SE-0345, the new shorthand optional unwrapping syntax removes unnecessary boilerplate while retaining clarity. Auto-formatting for this rule is implemented in https://github.com/nicklockwood/SwiftFormat/pull/1165.

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