Add ide-assist: flip_else_if

[A4-Tacks]

Flips two if-else-if branches.

Example

fn foo() {
    let n = if cond1 { 2 } else$0 if cond2 { 3 } else { 0 };
}

->

fn foo() {
    let n = if cond2 { 3 } else if cond1 { 2 } else { 0 };
}