Add rule to add a line break after the assignment operator before a multi-line if or switch expressions

[calda]

Summary

This PR proposes a new rule to add a line break after the assignment operator before a multi-line if or switch expression.

Autocorrection for this rule is implemented in the wrapMultilineConditionalAssignment SwiftFormat rule, added in nicklockwood/SwiftFormat#1574.

Reasoning

This makes it so that if and switch expressions always have the same "shape" as standard if and switch statements, where:

1. The if / switch keyword is always the left-most token on a dedicated line of code
2. The conditional branches are always to the right of and below the if / switch keyword.

This is most consistent with the formatting of if / switch statements in other contexts, and thus makes it easier to recognize that the code is using an if or switch expression at a glance.

This is independent of #250 and could be accepted even if we reject #250, but #250 depends on these changes given the discussion here.

Examples

// WRONG. Should have a line break after the `=`. 
let planetLocation = if let star = planet.star {
  "The \(star.name) system"
 } else {
  "Rogue planet"
}

// WRONG. The `=` should be on the line before the `if` / `switch` expression. 
let planetLocation 
  = if let star = planet.star {
    "The \(star.name) system"
  } else {
    "Rogue planet"
  }
  
// RIGHT 
let planetLocation = 
  if let star = planet.star {
    "The \(star.name) system"
  } else {
    "Rogue planet"
  }
  
// RIGHT 
let planetLocation = 
  if let star = planet.star {
    "The \(star.name) system"
  } else {
    "Rogue planet"
  }
  
// RIGHT
let planetType: PlanetType =
  switch planet {
  case .mercury, .venus, .earth, .mars:
    .terrestrial
  case .jupiter, .saturn, .uranus, .neptune:
    .gasGiant
  }
  
// ALSO RIGHT. A line break is not required because the declaration fits on a single line. 
let planet = if useFirst { mercury } else { neptune }

// ALSO RIGHT. A line break is always permitted if it helps with readability.
let planet =
  if useFirst { mercury } else { neptune }

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

[bachand]

Love it! Thanks @calda .

[bachand]

Nice catch!

[bachand]

Nice!