feat: offer block let fallback postfix complete

[A4-Tacks]

Related #5290 (comment)

@flodiebold Can this close the issue?

Um... it's a bit strange if it's inside the chain call,
e.g foo.bar().let.baz() -> {let $1 = foo.bar();$0}.baz()

Or only offer when the parent is a MatchArm?

Example

fn main() {
    match 2 {
        bar => bar.$0
    }
}

->

fn main() {
    match 2 {
        bar => {
            let $1 = bar;
            $0
        }
    }
}

[ChayimFriedman2]

I don't think we want this in general expressions, but inside a match arm it'll be nice to have both postfix and normal let (that complete the block as well), I've wanted this several times.