Rule Request: Prefer `if` and `switch` expressions

In version 5.9, Swift got support for if and switch expressions. A rule to enforce them where allowed should be possible.

In the first implementation, the rule does not need to be configurable. It should be opt-in and might support automatic rewriting.

Triggering:

func f(cond: Bool) -> Int {
    if cond {
        return 1
    } else {
        return 2
    }
}

func f(cond: Bool) {
    let r: Int
    if cond {
        r = 1
    } else {
        r = 2
    }
}

Non-triggering:

func f(cond: Bool) -> Int {
    if cond {
        // Nothing
    } else {
        return 2
    }
    return 1
}

Review SwiftLint's existing rule implementations and how opt-in rules express triggering and non-triggering cases. Use the Swift 5.9 examples in the issue as the initial acceptance criteria, including both return expressions and assigned values, and verify that the empty-branch example remains non-triggering.