Add ide-assist: merge_nested_else_if

[A4-Tacks]

This transforms if x {} else { if y {} } into if x {} else if y {},
This assist can only be applied with the cursor on else.

Example

fn main() {
    if x == 3 {
        true
    } $0else {
        if x == 2 {
            false
        } else {
            true
        }
    }
}

->

fn main() {
    if x == 3 {
        true
    } else if x == 2 {
        false
    } else {
        true
    }
}