feat: offer on match kw to merge all match arms

[A4-Tacks]

Close #20323

Example

fn main() {
    let y = $0match X::A {
        X::A => 1i32,
        X::B => 1i32,
        X::C => 2i32,
        X::D => 3i32,
        X::E => 3i32,
    }
}

Before this PR

Assist not applicable

After this PR

fn main() {
    let y = match X::A {
        X::A | X::B => 1i32,
        X::C => 2i32,
        X::D | X::E => 3i32,
    }
}

[rustbot]

This PR was rebased onto a different master commit. Here's a range-diff highlighting what actually changed.

Rebasing is a normal part of keeping PRs up to date, so no action is needed—this note is just to help reviewers.

[ChayimFriedman2]

I remain in the opinion that this is not worth it, even without a new assist.