Stabilize $$ and ${ignore} in Rust 1.62.0

An attempt to stabilize "$$" and "${ignore}" is available at https://github.com/rust-lang/rust/pull/95860.

If approved, the following will be possible in stable compiler versions:

macro_rules! count {
    ( $( $i:stmt ),* ) => {{
        0 $( + 1 ${ignore(i)} )*
    }};
}

fn main() {
    dbg!(count!(if true {} else {}, let _: () = (), || false));
}