fix: add else keyword completion after let statements

[A4-Tacks]

Example

fn foo() {
    let _ = 2 el$0
}

->

fn foo() {
    let _ = 2 else {
        $0
    };
}

---

And fix the no completion semicolon for else in incomplete LetStmt of #20518

fn foo() {
    let _ = if true {} el$0
}

Before this PR:

fn foo() {
    let _ = if true {} else {
        $0
    }
}

After this PR:

fn foo() {
    let _ = if true {} else {
        $0
    };
}

[A4-Tacks]

Closed as completed for #20620

[A4-Tacks]

Sorry, I mistook this for an issue