lint: add if/else -> switch suggesting checker

[quasilyte]

If there is a chain of if/else, suggest switch as alternative.

These two functions should trigger the warning:

func cond1(x int) string {
	if x == 0 {
		return "zero"
	} else if x < 0 {
		return "negative"
	} else {
		return "positive"
	}
}

func cond2(x int) string {
	if x == 0 {
		return "zero"
	} else if x < 0 {
		return "negative"
	}
	return "positive"
}

Suggested alternative is:

func condSuggested(x int) string {
	switch {
	case x == 0:
		return "zero"
	case x < 0:
		return "negative"
	default:
		return "positive"
	}
}

TODO: find out if if/else is ever more readable or wanted instead of switch.
Maybe init statements can complicate things.