add `@__FUNCTION__` macro

https://github.com/JuliaLang/julia/pull/58940 The macro implemented here differs slightly because it needs to refer to the internal variable `var"#self#"`, rather than the new `Expr(:thisfunction)` directive, as the later required parser changes to expand compatibility with callable structs. However, the only change this means in practice is that the Compat version will throw an error in cases where the Julia 1.13.0-DEV.880 version would work. So there would not be any silent differences; only very obvious ones. I have mentioned these cases on the Compat version docstring.