Neocaml 0.6: Opam, Dune, and More

When I released neocaml 0.1 last month, I thought I was more or less done with the (main) features for the foreseeable future. The original scope was deliberately small — a couple of Tree-sitter-powered OCaml major modes (for .ml and .mli), a REPL integration, and not much else. I was quite happy with how things turned out and figured the next steps would be mostly polish and bug fixes.