[enhancement] dslx_fmt let module-level attributes come after license banner

[cdleary]

What's hard to do? (limit 100 words)

Right now it seems like dslx_fmt always places a module attribute like #![feature(use_syntax)] at the very top of the file above a module level comment -- in all other files the header comment is at the top, so seems like it could be good to have the #! underneath the license header comment. In the past I've written hacky scripts that check for the license header "at the very start or after a shebang line", not sure if other things in the universe assume it'd be at the start.

Current best alternative workaround (limit 100 words)

Put the attribute above the license header comment block when you have autoformatting on.

Your view of the "best case XLS enhancement" (limit 100 words)

Module-level attributes could auto-format after a top of file comment block like a license header.