Add a new concat metavar expr Revival of #111930 Giving it another try now that #117050 was merged. With the new rules, meta-variable expressions must be referenced with a dollar sign (`$`) and this can cause misunderstands with `$concat`. ```rust macro_rules! foo { ( $bar:ident ) => { const ${concat(VAR, bar)}: i32 = 1; }; } // Will produce `VARbar` instead of `VAR_123` foo!(_123); ``` In other words, forgetting the dollar symbol can produce undesired outputs. cc #29599 cc https://github.com/rust-lang/rust/issues/124225