Add rule to prefer if expressions over ternary operators in single-expression return values

[calda]

Summary

Prefer if expressions over ternary operators for single-expression return values. Use ternary operators for conditions nested in other expressions, such as SwiftUI modifier conditions. Generally prefer if expressions for assignments after = operators.

// WRONG
var destination: Planet {
  spaceship.hasWarpDrive
    ? .proximaCentauri
    : .mars
}

// RIGHT
var destination: Planet {
  if spaceship.hasWarpDrive {
    .proximaCentauri
  } else {
    .mars
  }
}

// ALSO RIGHT. Use ternaries for conditions nested in other expressions,
// like SwiftUI modifier conditions.
Image(.spaceship)
  .foregroundStyle(spaceship.velocity > 0 ? .orange : .blue)

Reasoning

If expressions are easier to read than ternary operators, especially when nesting multiple conditions. If expressions are more modern and support more functionality, like if let conditions.

[calda]

@copilot, please add an "ALSO RIGHT" for a computed var ternary where the ternary is on a single line rather than wrapped. Single-line ternaries are permitted. Use the color example.

[Copilot]

Addressed in f245a80.

[calda]

@copilot add a ### Why like other rules with: If expressions are more readable than ternary expressions, especially for multiple nested conditions or multi-line values.

[Copilot]

Addressed in 84c82be.

[calda]

@copilot, wrap after the {, but the ternary itself can be on a single line.

[Copilot]

Addressed in 3be06c7.