Add rule to add a linebreak before the else keyword in a multi-line guard statement

[calda]

Summary

This PR proposes a new rule to add a linebreak before the else keyword in a multi-line guard statement.

// WRONG -- else should be on its own line for multi-line guard statements
guard
  let galaxy,
  galaxy.name == "Milky Way" else
{ … }

// WRONG -- else should be on the same line for single-line guard statements
guard let galaxy
else { … }

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

// RIGHT
guard let galaxy else {
  …
}

Reasoning

This was first suggested back in #108 (comment). This is also how the examples in the "wrap multi-line conditionals" rule description are formatted.

Placing the else on its own line makes it more visually distinct, and prevents it from blending in with the last statement.

We previously didn't enforce this through the formatter since the SwiftFormat rule for this didn't match our requirements very well. As of nicklockwood/SwiftFormat#1447 the SwiftFormat rule implementation works well for us.

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