Support else completion for more expressions

[A4-Tacks]

• Support else completion in ArrayExpr, ReturnExpr and PrefixExpr etc
• Support else completion after MatchArm expression

Before this PR, the else branch could not be completed in most expressions

Example

fn foo() -> [i32; 1] {
    [if true {
        2
    } $0]
}

->

fn foo() -> [i32; 1] {
    [if true {
        2
    } else {
        $0
    }]
}

---

fn foo() -> i32 {
    match () {
        () => if true {
            2
        } $0
    }
}

->

fn foo() -> i32 {
    match () {
        () => if true {
            2
        } else {
            $0
        }
    }
}